Add native Windows binary for bison.

bison/src/bison/2.4.1/bison-2.4.1-src/doc/bison.texinfo

+\input texinfo @c -*-texinfo-*-
+@comment %**start of header
+@setfilename bison.info
+@include version.texi
+@settitle Bison @value{VERSION}
+@setchapternewpage odd
+
+@finalout
+
+@c SMALL BOOK version
+@c This edition has been formatted so that you can format and print it in
+@c the smallbook format.
+@c @smallbook
+
+@c Set following if you want to document %default-prec and %no-default-prec.
+@c This feature is experimental and may change in future Bison versions.
+@c @set defaultprec
+
+@ifnotinfo
+@syncodeindex fn cp
+@syncodeindex vr cp
+@syncodeindex tp cp
+@end ifnotinfo
+@ifinfo
+@synindex fn cp
+@synindex vr cp
+@synindex tp cp
+@end ifinfo
+@comment %**end of header
+
+@copying
+
+This manual (@value{UPDATED}) is for @acronym{GNU} Bison (version
+@value{VERSION}), the @acronym{GNU} parser generator.
+
+Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998,
+1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 Free Software
+Foundation, Inc.
+
+@quotation
+Permission is granted to copy, distribute and/or modify this document
+under the terms of the @acronym{GNU} Free Documentation License,
+Version 1.2 or any later version published by the Free Software
+Foundation; with no Invariant Sections, with the Front-Cover texts
+being ``A @acronym{GNU} Manual,'' and with the Back-Cover Texts as in
+(a) below.  A copy of the license is included in the section entitled
+``@acronym{GNU} Free Documentation License.''
+
+(a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
+modify this @acronym{GNU} manual.  Buying copies from the @acronym{FSF}
+supports it in developing @acronym{GNU} and promoting software
+freedom.''
+@end quotation
+@end copying
+
+@dircategory Software development
+@direntry
+* bison: (bison).       @acronym{GNU} parser generator (Yacc replacement).
+@end direntry
+
+@titlepage
+@title Bison
+@subtitle The Yacc-compatible Parser Generator
+@subtitle @value{UPDATED}, Bison Version @value{VERSION}
+
+@author by Charles Donnelly and Richard Stallman
+
+@page
+@vskip 0pt plus 1filll
+@insertcopying
+@sp 2
+Published by the Free Software Foundation @*
+51 Franklin Street, Fifth Floor @*
+Boston, MA  02110-1301  USA @*
+Printed copies are available from the Free Software Foundation.@*
+@acronym{ISBN} 1-882114-44-2
+@sp 2
+Cover art by Etienne Suvasa.
+@end titlepage
+
+@contents
+
+@ifnottex
+@node Top
+@top Bison
+@insertcopying
+@end ifnottex
+
+@menu
+* Introduction::
+* Conditions::
+* Copying::             The @acronym{GNU} General Public License says
+                          how you can copy and share Bison.
+
+Tutorial sections:
+* Concepts::            Basic concepts for understanding Bison.
+* Examples::            Three simple explained examples of using Bison.
+
+Reference sections:
+* Grammar File::        Writing Bison declarations and rules.
+* Interface::           C-language interface to the parser function @code{yyparse}.
+* Algorithm::           How the Bison parser works at run-time.
+* Error Recovery::      Writing rules for error recovery.
+* Context Dependency::  What to do if your language syntax is too
+                          messy for Bison to handle straightforwardly.
+* Debugging::           Understanding or debugging Bison parsers.
+* Invocation::          How to run Bison (to produce the parser source file).
+* Other Languages::     Creating C++ and Java parsers.
+* FAQ::                 Frequently Asked Questions
+* Table of Symbols::    All the keywords of the Bison language are explained.
+* Glossary::            Basic concepts are explained.
+* Copying This Manual:: License for copying this manual.
+* Index::               Cross-references to the text.
+
+@detailmenu
+ --- The Detailed Node Listing ---
+
+The Concepts of Bison
+
+* Language and Grammar:: Languages and context-free grammars,
+                           as mathematical ideas.
+* Grammar in Bison::     How we represent grammars for Bison's sake.
+* Semantic Values::      Each token or syntactic grouping can have
+                           a semantic value (the value of an integer,
+                           the name of an identifier, etc.).
+* Semantic Actions::     Each rule can have an action containing C code.
+* GLR Parsers::          Writing parsers for general context-free languages.
+* Locations Overview::   Tracking Locations.
+* Bison Parser::         What are Bison's input and output,
+                           how is the output used?
+* Stages::               Stages in writing and running Bison grammars.
+* Grammar Layout::       Overall structure of a Bison grammar file.
+
+Writing @acronym{GLR} Parsers
+
+* Simple GLR Parsers::     Using @acronym{GLR} parsers on unambiguous grammars.
+* Merging GLR Parses::     Using @acronym{GLR} parsers to resolve ambiguities.
+* GLR Semantic Actions::   Deferred semantic actions have special concerns.
+* Compiler Requirements::  @acronym{GLR} parsers require a modern C compiler.
+
+Examples
+
+* RPN Calc::               Reverse polish notation calculator;
+                             a first example with no operator precedence.
+* Infix Calc::             Infix (algebraic) notation calculator.
+                             Operator precedence is introduced.
+* Simple Error Recovery::  Continuing after syntax errors.
+* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$.
+* Multi-function Calc::    Calculator with memory and trig functions.
+                             It uses multiple data-types for semantic values.
+* Exercises::              Ideas for improving the multi-function calculator.
+
+Reverse Polish Notation Calculator
+
+* Rpcalc Declarations::    Prologue (declarations) for rpcalc.
+* Rpcalc Rules::           Grammar Rules for rpcalc, with explanation.
+* Rpcalc Lexer::           The lexical analyzer.
+* Rpcalc Main::            The controlling function.
+* Rpcalc Error::           The error reporting function.
+* Rpcalc Generate::        Running Bison on the grammar file.
+* Rpcalc Compile::         Run the C compiler on the output code.
+
+Grammar Rules for @code{rpcalc}
+
+* Rpcalc Input::
+* Rpcalc Line::
+* Rpcalc Expr::
+
+Location Tracking Calculator: @code{ltcalc}
+
+* Ltcalc Declarations::    Bison and C declarations for ltcalc.
+* Ltcalc Rules::           Grammar rules for ltcalc, with explanations.
+* Ltcalc Lexer::           The lexical analyzer.
+
+Multi-Function Calculator: @code{mfcalc}
+
+* Mfcalc Declarations::    Bison declarations for multi-function calculator.
+* Mfcalc Rules::           Grammar rules for the calculator.
+* Mfcalc Symbol Table::    Symbol table management subroutines.
+
+Bison Grammar Files
+
+* Grammar Outline::   Overall layout of the grammar file.
+* Symbols::           Terminal and nonterminal symbols.
+* Rules::             How to write grammar rules.
+* Recursion::         Writing recursive rules.
+* Semantics::         Semantic values and actions.
+* Locations::         Locations and actions.
+* Declarations::      All kinds of Bison declarations are described here.
+* Multiple Parsers::  Putting more than one Bison parser in one program.
+
+Outline of a Bison Grammar
+
+* Prologue::              Syntax and usage of the prologue.
+* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
+* Bison Declarations::    Syntax and usage of the Bison declarations section.
+* Grammar Rules::         Syntax and usage of the grammar rules section.
+* Epilogue::              Syntax and usage of the epilogue.
+
+Defining Language Semantics
+
+* Value Type::        Specifying one data type for all semantic values.
+* Multiple Types::    Specifying several alternative data types.
+* Actions::           An action is the semantic definition of a grammar rule.
+* Action Types::      Specifying data types for actions to operate on.
+* Mid-Rule Actions::  Most actions go at the end of a rule.
+                      This says when, why and how to use the exceptional
+                        action in the middle of a rule.
+
+Tracking Locations
+
+* Location Type::               Specifying a data type for locations.
+* Actions and Locations::       Using locations in actions.
+* Location Default Action::     Defining a general way to compute locations.
+
+Bison Declarations
+
+* Require Decl::      Requiring a Bison version.
+* Token Decl::        Declaring terminal symbols.
+* Precedence Decl::   Declaring terminals with precedence and associativity.
+* Union Decl::        Declaring the set of all semantic value types.
+* Type Decl::         Declaring the choice of type for a nonterminal symbol.
+* Initial Action Decl::  Code run before parsing starts.
+* Destructor Decl::   Declaring how symbols are freed.
+* Expect Decl::       Suppressing warnings about parsing conflicts.
+* Start Decl::        Specifying the start symbol.
+* Pure Decl::         Requesting a reentrant parser.
+* Push Decl::         Requesting a push parser.
+* Decl Summary::      Table of all Bison declarations.
+
+Parser C-Language Interface
+
+* Parser Function::         How to call @code{yyparse} and what it returns.
+* Push Parser Function::    How to call @code{yypush_parse} and what it returns.
+* Pull Parser Function::    How to call @code{yypull_parse} and what it returns.
+* Parser Create Function::  How to call @code{yypstate_new} and what it returns.
+* Parser Delete Function::  How to call @code{yypstate_delete} and what it returns.
+* Lexical::                 You must supply a function @code{yylex}
+                              which reads tokens.
+* Error Reporting::         You must supply a function @code{yyerror}.
+* Action Features::         Special features for use in actions.
+* Internationalization::    How to let the parser speak in the user's
+                              native language.
+
+The Lexical Analyzer Function @code{yylex}
+
+* Calling Convention::  How @code{yyparse} calls @code{yylex}.
+* Token Values::        How @code{yylex} must return the semantic value
+                          of the token it has read.
+* Token Locations::     How @code{yylex} must return the text location
+                          (line number, etc.) of the token, if the
+                          actions want that.
+* Pure Calling::        How the calling convention differs in a pure parser
+                          (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}).
+
+The Bison Parser Algorithm
+
+* Lookahead::         Parser looks one token ahead when deciding what to do.
+* Shift/Reduce::      Conflicts: when either shifting or reduction is valid.
+* Precedence::        Operator precedence works by resolving conflicts.
+* Contextual Precedence::  When an operator's precedence depends on context.
+* Parser States::     The parser is a finite-state-machine with stack.
+* Reduce/Reduce::     When two rules are applicable in the same situation.
+* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
+* Generalized LR Parsing::  Parsing arbitrary context-free grammars.
+* Memory Management:: What happens when memory is exhausted.  How to avoid it.
+
+Operator Precedence
+
+* Why Precedence::    An example showing why precedence is needed.
+* Using Precedence::  How to specify precedence in Bison grammars.
+* Precedence Examples::  How these features are used in the previous example.
+* How Precedence::    How they work.
+
+Handling Context Dependencies
+
+* Semantic Tokens::   Token parsing can depend on the semantic context.
+* Lexical Tie-ins::   Token parsing can depend on the syntactic context.
+* Tie-in Recovery::   Lexical tie-ins have implications for how
+                        error recovery rules must be written.
+
+Debugging Your Parser
+
+* Understanding::     Understanding the structure of your parser.
+* Tracing::           Tracing the execution of your parser.
+
+Invoking Bison
+
+* Bison Options::     All the options described in detail,
+                        in alphabetical order by short options.
+* Option Cross Key::  Alphabetical list of long options.
+* Yacc Library::      Yacc-compatible @code{yylex} and @code{main}.
+
+Parsers Written In Other Languages
+
+* C++ Parsers::                 The interface to generate C++ parser classes
+* Java Parsers::                The interface to generate Java parser classes
+
+C++ Parsers
+
+* C++ Bison Interface::         Asking for C++ parser generation
+* C++ Semantic Values::         %union vs. C++
+* C++ Location Values::         The position and location classes
+* C++ Parser Interface::        Instantiating and running the parser
+* C++ Scanner Interface::       Exchanges between yylex and parse
+* A Complete C++ Example::      Demonstrating their use
+
+A Complete C++ Example
+
+* Calc++ --- C++ Calculator::   The specifications
+* Calc++ Parsing Driver::       An active parsing context
+* Calc++ Parser::               A parser class
+* Calc++ Scanner::              A pure C++ Flex scanner
+* Calc++ Top Level::            Conducting the band
+
+Java Parsers
+
+* Java Bison Interface::        Asking for Java parser generation
+* Java Semantic Values::        %type and %token vs. Java
+* Java Location Values::        The position and location classes
+* Java Parser Interface::       Instantiating and running the parser
+* Java Scanner Interface::      Specifying the scanner for the parser
+* Java Action Features::        Special features for use in actions
+* Java Differences::            Differences between C/C++ and Java Grammars
+* Java Declarations Summary::   List of Bison declarations used with Java
+
+Frequently Asked Questions
+
+* Memory Exhausted::            Breaking the Stack Limits
+* How Can I Reset the Parser::  @code{yyparse} Keeps some State
+* Strings are Destroyed::       @code{yylval} Loses Track of Strings
+* Implementing Gotos/Loops::    Control Flow in the Calculator
+* Multiple start-symbols::      Factoring closely related grammars
+* Secure?  Conform?::           Is Bison @acronym{POSIX} safe?
+* I can't build Bison::         Troubleshooting
+* Where can I find help?::      Troubleshouting
+* Bug Reports::                 Troublereporting
+* More Languages::              Parsers in C++, Java, and so on
+* Beta Testing::                Experimenting development versions
+* Mailing Lists::               Meeting other Bison users
+
+Copying This Manual
+
+* Copying This Manual::         License for copying this manual.
+
+@end detailmenu
+@end menu
+
+@node Introduction
+@unnumbered Introduction
+@cindex introduction
+
+@dfn{Bison} is a general-purpose parser generator that converts an
+annotated context-free grammar into an @acronym{LALR}(1) or
+@acronym{GLR} parser for that grammar.  Once you are proficient with
+Bison, you can use it to develop a wide range of language parsers, from those
+used in simple desk calculators to complex programming languages.
+
+Bison is upward compatible with Yacc: all properly-written Yacc grammars
+ought to work with Bison with no change.  Anyone familiar with Yacc
+should be able to use Bison with little trouble.  You need to be fluent in
+C or C++ programming in order to use Bison or to understand this manual.
+
+We begin with tutorial chapters that explain the basic concepts of using
+Bison and show three explained examples, each building on the last.  If you
+don't know Bison or Yacc, start by reading these chapters.  Reference
+chapters follow which describe specific aspects of Bison in detail.
+
+Bison was written primarily by Robert Corbett; Richard Stallman made it
+Yacc-compatible.  Wilfred Hansen of Carnegie Mellon University added
+multi-character string literals and other features.
+
+This edition corresponds to version @value{VERSION} of Bison.
+
+@node Conditions
+@unnumbered Conditions for Using Bison
+
+The distribution terms for Bison-generated parsers permit using the
+parsers in nonfree programs.  Before Bison version 2.2, these extra
+permissions applied only when Bison was generating @acronym{LALR}(1)
+parsers in C@.  And before Bison version 1.24, Bison-generated
+parsers could be used only in programs that were free software.
+
+The other @acronym{GNU} programming tools, such as the @acronym{GNU} C
+compiler, have never
+had such a requirement.  They could always be used for nonfree
+software.  The reason Bison was different was not due to a special
+policy decision; it resulted from applying the usual General Public
+License to all of the Bison source code.
+
+The output of the Bison utility---the Bison parser file---contains a
+verbatim copy of a sizable piece of Bison, which is the code for the
+parser's implementation.  (The actions from your grammar are inserted
+into this implementation at one point, but most of the rest of the
+implementation is not changed.)  When we applied the @acronym{GPL}
+terms to the skeleton code for the parser's implementation,
+the effect was to restrict the use of Bison output to free software.
+
+We didn't change the terms because of sympathy for people who want to
+make software proprietary.  @strong{Software should be free.}  But we
+concluded that limiting Bison's use to free software was doing little to
+encourage people to make other software free.  So we decided to make the
+practical conditions for using Bison match the practical conditions for
+using the other @acronym{GNU} tools.
+
+This exception applies when Bison is generating code for a parser.
+You can tell whether the exception applies to a Bison output file by
+inspecting the file for text beginning with ``As a special
+exception@dots{}''.  The text spells out the exact terms of the
+exception.
+
+@node Copying
+@unnumbered GNU GENERAL PUBLIC LICENSE
+@include gpl-3.0.texi
+
+@node Concepts
+@chapter The Concepts of Bison
+
+This chapter introduces many of the basic concepts without which the
+details of Bison will not make sense.  If you do not already know how to
+use Bison or Yacc, we suggest you start by reading this chapter carefully.
+
+@menu
+* Language and Grammar:: Languages and context-free grammars,
+                           as mathematical ideas.
+* Grammar in Bison::     How we represent grammars for Bison's sake.
+* Semantic Values::      Each token or syntactic grouping can have
+                           a semantic value (the value of an integer,
+                           the name of an identifier, etc.).
+* Semantic Actions::     Each rule can have an action containing C code.
+* GLR Parsers::          Writing parsers for general context-free languages.
+* Locations Overview::   Tracking Locations.
+* Bison Parser::         What are Bison's input and output,
+                           how is the output used?
+* Stages::               Stages in writing and running Bison grammars.
+* Grammar Layout::       Overall structure of a Bison grammar file.
+@end menu
+
+@node Language and Grammar
+@section Languages and Context-Free Grammars
+
+@cindex context-free grammar
+@cindex grammar, context-free
+In order for Bison to parse a language, it must be described by a
+@dfn{context-free grammar}.  This means that you specify one or more
+@dfn{syntactic groupings} and give rules for constructing them from their
+parts.  For example, in the C language, one kind of grouping is called an
+`expression'.  One rule for making an expression might be, ``An expression
+can be made of a minus sign and another expression''.  Another would be,
+``An expression can be an integer''.  As you can see, rules are often
+recursive, but there must be at least one rule which leads out of the
+recursion.
+
+@cindex @acronym{BNF}
+@cindex Backus-Naur form
+The most common formal system for presenting such rules for humans to read
+is @dfn{Backus-Naur Form} or ``@acronym{BNF}'', which was developed in
+order to specify the language Algol 60.  Any grammar expressed in
+@acronym{BNF} is a context-free grammar.  The input to Bison is
+essentially machine-readable @acronym{BNF}.
+
+@cindex @acronym{LALR}(1) grammars
+@cindex @acronym{LR}(1) grammars
+There are various important subclasses of context-free grammar.  Although it
+can handle almost all context-free grammars, Bison is optimized for what
+are called @acronym{LALR}(1) grammars.
+In brief, in these grammars, it must be possible to
+tell how to parse any portion of an input string with just a single
+token of lookahead.  Strictly speaking, that is a description of an
+@acronym{LR}(1) grammar, and @acronym{LALR}(1) involves additional
+restrictions that are
+hard to explain simply; but it is rare in actual practice to find an
+@acronym{LR}(1) grammar that fails to be @acronym{LALR}(1).
+@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}, for
+more information on this.
+
+@cindex @acronym{GLR} parsing
+@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
+@cindex ambiguous grammars
+@cindex nondeterministic parsing
+
+Parsers for @acronym{LALR}(1) grammars are @dfn{deterministic}, meaning
+roughly that the next grammar rule to apply at any point in the input is
+uniquely determined by the preceding input and a fixed, finite portion
+(called a @dfn{lookahead}) of the remaining input.  A context-free
+grammar can be @dfn{ambiguous}, meaning that there are multiple ways to
+apply the grammar rules to get the same inputs.  Even unambiguous
+grammars can be @dfn{nondeterministic}, meaning that no fixed
+lookahead always suffices to determine the next grammar rule to apply.
+With the proper declarations, Bison is also able to parse these more
+general context-free grammars, using a technique known as @acronym{GLR}
+parsing (for Generalized @acronym{LR}).  Bison's @acronym{GLR} parsers
+are able to handle any context-free grammar for which the number of
+possible parses of any given string is finite.
+
+@cindex symbols (abstract)
+@cindex token
+@cindex syntactic grouping
+@cindex grouping, syntactic
+In the formal grammatical rules for a language, each kind of syntactic
+unit or grouping is named by a @dfn{symbol}.  Those which are built by
+grouping smaller constructs according to grammatical rules are called
+@dfn{nonterminal symbols}; those which can't be subdivided are called
+@dfn{terminal symbols} or @dfn{token types}.  We call a piece of input
+corresponding to a single terminal symbol a @dfn{token}, and a piece
+corresponding to a single nonterminal symbol a @dfn{grouping}.
+
+We can use the C language as an example of what symbols, terminal and
+nonterminal, mean.  The tokens of C are identifiers, constants (numeric
+and string), and the various keywords, arithmetic operators and
+punctuation marks.  So the terminal symbols of a grammar for C include
+`identifier', `number', `string', plus one symbol for each keyword,
+operator or punctuation mark: `if', `return', `const', `static', `int',
+`char', `plus-sign', `open-brace', `close-brace', `comma' and many more.
+(These tokens can be subdivided into characters, but that is a matter of
+lexicography, not grammar.)
+
+Here is a simple C function subdivided into tokens:
+
+@ifinfo
+@example
+int             /* @r{keyword `int'} */
+square (int x)  /* @r{identifier, open-paren, keyword `int',}
+                   @r{identifier, close-paren} */
+@{               /* @r{open-brace} */
+  return x * x; /* @r{keyword `return', identifier, asterisk,}
+                   @r{identifier, semicolon} */
+@}               /* @r{close-brace} */
+@end example
+@end ifinfo
+@ifnotinfo
+@example
+int             /* @r{keyword `int'} */
+square (int x)  /* @r{identifier, open-paren, keyword `int', identifier, close-paren} */
+@{               /* @r{open-brace} */
+  return x * x; /* @r{keyword `return', identifier, asterisk, identifier, semicolon} */
+@}               /* @r{close-brace} */
+@end example
+@end ifnotinfo
+
+The syntactic groupings of C include the expression, the statement, the
+declaration, and the function definition.  These are represented in the
+grammar of C by nonterminal symbols `expression', `statement',
+`declaration' and `function definition'.  The full grammar uses dozens of
+additional language constructs, each with its own nonterminal symbol, in
+order to express the meanings of these four.  The example above is a
+function definition; it contains one declaration, and one statement.  In
+the statement, each @samp{x} is an expression and so is @samp{x * x}.
+
+Each nonterminal symbol must have grammatical rules showing how it is made
+out of simpler constructs.  For example, one kind of C statement is the
+@code{return} statement; this would be described with a grammar rule which
+reads informally as follows:
+
+@quotation
+A `statement' can be made of a `return' keyword, an `expression' and a
+`semicolon'.
+@end quotation
+
+@noindent
+There would be many other rules for `statement', one for each kind of
+statement in C.
+
+@cindex start symbol
+One nonterminal symbol must be distinguished as the special one which
+defines a complete utterance in the language.  It is called the @dfn{start
+symbol}.  In a compiler, this means a complete input program.  In the C
+language, the nonterminal symbol `sequence of definitions and declarations'
+plays this role.
+
+For example, @samp{1 + 2} is a valid C expression---a valid part of a C
+program---but it is not valid as an @emph{entire} C program.  In the
+context-free grammar of C, this follows from the fact that `expression' is
+not the start symbol.
+
+The Bison parser reads a sequence of tokens as its input, and groups the
+tokens using the grammar rules.  If the input is valid, the end result is
+that the entire token sequence reduces to a single grouping whose symbol is
+the grammar's start symbol.  If we use a grammar for C, the entire input
+must be a `sequence of definitions and declarations'.  If not, the parser
+reports a syntax error.
+
+@node Grammar in Bison
+@section From Formal Rules to Bison Input
+@cindex Bison grammar
+@cindex grammar, Bison
+@cindex formal grammar
+
+A formal grammar is a mathematical construct.  To define the language
+for Bison, you must write a file expressing the grammar in Bison syntax:
+a @dfn{Bison grammar} file.  @xref{Grammar File, ,Bison Grammar Files}.
+
+A nonterminal symbol in the formal grammar is represented in Bison input
+as an identifier, like an identifier in C@.  By convention, it should be
+in lower case, such as @code{expr}, @code{stmt} or @code{declaration}.
+
+The Bison representation for a terminal symbol is also called a @dfn{token
+type}.  Token types as well can be represented as C-like identifiers.  By
+convention, these identifiers should be upper case to distinguish them from
+nonterminals: for example, @code{INTEGER}, @code{IDENTIFIER}, @code{IF} or
+@code{RETURN}.  A terminal symbol that stands for a particular keyword in
+the language should be named after that keyword converted to upper case.
+The terminal symbol @code{error} is reserved for error recovery.
+@xref{Symbols}.
+
+A terminal symbol can also be represented as a character literal, just like
+a C character constant.  You should do this whenever a token is just a
+single character (parenthesis, plus-sign, etc.): use that same character in
+a literal as the terminal symbol for that token.
+
+A third way to represent a terminal symbol is with a C string constant
+containing several characters.  @xref{Symbols}, for more information.
+
+The grammar rules also have an expression in Bison syntax.  For example,
+here is the Bison rule for a C @code{return} statement.  The semicolon in
+quotes is a literal character token, representing part of the C syntax for
+the statement; the naked semicolon, and the colon, are Bison punctuation
+used in every rule.
+
+@example
+stmt:   RETURN expr ';'
+        ;
+@end example
+
+@noindent
+@xref{Rules, ,Syntax of Grammar Rules}.
+
+@node Semantic Values
+@section Semantic Values
+@cindex semantic value
+@cindex value, semantic
+
+A formal grammar selects tokens only by their classifications: for example,
+if a rule mentions the terminal symbol `integer constant', it means that
+@emph{any} integer constant is grammatically valid in that position.  The
+precise value of the constant is irrelevant to how to parse the input: if
+@samp{x+4} is grammatical then @samp{x+1} or @samp{x+3989} is equally
+grammatical.
+
+But the precise value is very important for what the input means once it is
+parsed.  A compiler is useless if it fails to distinguish between 4, 1 and
+3989 as constants in the program!  Therefore, each token in a Bison grammar
+has both a token type and a @dfn{semantic value}.  @xref{Semantics,
+,Defining Language Semantics},
+for details.
+
+The token type is a terminal symbol defined in the grammar, such as
+@code{INTEGER}, @code{IDENTIFIER} or @code{','}.  It tells everything
+you need to know to decide where the token may validly appear and how to
+group it with other tokens.  The grammar rules know nothing about tokens
+except their types.
+
+The semantic value has all the rest of the information about the
+meaning of the token, such as the value of an integer, or the name of an
+identifier.  (A token such as @code{','} which is just punctuation doesn't
+need to have any semantic value.)
+
+For example, an input token might be classified as token type
+@code{INTEGER} and have the semantic value 4.  Another input token might
+have the same token type @code{INTEGER} but value 3989.  When a grammar
+rule says that @code{INTEGER} is allowed, either of these tokens is
+acceptable because each is an @code{INTEGER}.  When the parser accepts the
+token, it keeps track of the token's semantic value.
+
+Each grouping can also have a semantic value as well as its nonterminal
+symbol.  For example, in a calculator, an expression typically has a
+semantic value that is a number.  In a compiler for a programming
+language, an expression typically has a semantic value that is a tree
+structure describing the meaning of the expression.
+
+@node Semantic Actions
+@section Semantic Actions
+@cindex semantic actions
+@cindex actions, semantic
+
+In order to be useful, a program must do more than parse input; it must
+also produce some output based on the input.  In a Bison grammar, a grammar
+rule can have an @dfn{action} made up of C statements.  Each time the
+parser recognizes a match for that rule, the action is executed.
+@xref{Actions}.
+
+Most of the time, the purpose of an action is to compute the semantic value
+of the whole construct from the semantic values of its parts.  For example,
+suppose we have a rule which says an expression can be the sum of two
+expressions.  When the parser recognizes such a sum, each of the
+subexpressions has a semantic value which describes how it was built up.
+The action for this rule should create a similar sort of value for the
+newly recognized larger expression.
+
+For example, here is a rule that says an expression can be the sum of
+two subexpressions:
+
+@example
+expr: expr '+' expr   @{ $$ = $1 + $3; @}
+        ;
+@end example
+
+@noindent
+The action says how to produce the semantic value of the sum expression
+from the values of the two subexpressions.
+
+@node GLR Parsers
+@section Writing @acronym{GLR} Parsers
+@cindex @acronym{GLR} parsing
+@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
+@findex %glr-parser
+@cindex conflicts
+@cindex shift/reduce conflicts
+@cindex reduce/reduce conflicts
+
+In some grammars, Bison's standard
+@acronym{LALR}(1) parsing algorithm cannot decide whether to apply a
+certain grammar rule at a given point.  That is, it may not be able to
+decide (on the basis of the input read so far) which of two possible
+reductions (applications of a grammar rule) applies, or whether to apply
+a reduction or read more of the input and apply a reduction later in the
+input.  These are known respectively as @dfn{reduce/reduce} conflicts
+(@pxref{Reduce/Reduce}), and @dfn{shift/reduce} conflicts
+(@pxref{Shift/Reduce}).
+
+To use a grammar that is not easily modified to be @acronym{LALR}(1), a
+more general parsing algorithm is sometimes necessary.  If you include
+@code{%glr-parser} among the Bison declarations in your file
+(@pxref{Grammar Outline}), the result is a Generalized @acronym{LR}
+(@acronym{GLR}) parser.  These parsers handle Bison grammars that
+contain no unresolved conflicts (i.e., after applying precedence
+declarations) identically to @acronym{LALR}(1) parsers.  However, when
+faced with unresolved shift/reduce and reduce/reduce conflicts,
+@acronym{GLR} parsers use the simple expedient of doing both,
+effectively cloning the parser to follow both possibilities.  Each of
+the resulting parsers can again split, so that at any given time, there
+can be any number of possible parses being explored.  The parsers
+proceed in lockstep; that is, all of them consume (shift) a given input
+symbol before any of them proceed to the next.  Each of the cloned
+parsers eventually meets one of two possible fates: either it runs into
+a parsing error, in which case it simply vanishes, or it merges with
+another parser, because the two of them have reduced the input to an
+identical set of symbols.
+
+During the time that there are multiple parsers, semantic actions are
+recorded, but not performed.  When a parser disappears, its recorded
+semantic actions disappear as well, and are never performed.  When a
+reduction makes two parsers identical, causing them to merge, Bison
+records both sets of semantic actions.  Whenever the last two parsers
+merge, reverting to the single-parser case, Bison resolves all the
+outstanding actions either by precedences given to the grammar rules
+involved, or by performing both actions, and then calling a designated
+user-defined function on the resulting values to produce an arbitrary
+merged result.
+
+@menu
+* Simple GLR Parsers::     Using @acronym{GLR} parsers on unambiguous grammars.
+* Merging GLR Parses::     Using @acronym{GLR} parsers to resolve ambiguities.
+* GLR Semantic Actions::   Deferred semantic actions have special concerns.
+* Compiler Requirements::  @acronym{GLR} parsers require a modern C compiler.
+@end menu
+
+@node Simple GLR Parsers
+@subsection Using @acronym{GLR} on Unambiguous Grammars
+@cindex @acronym{GLR} parsing, unambiguous grammars
+@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, unambiguous grammars
+@findex %glr-parser
+@findex %expect-rr
+@cindex conflicts
+@cindex reduce/reduce conflicts
+@cindex shift/reduce conflicts
+
+In the simplest cases, you can use the @acronym{GLR} algorithm
+to parse grammars that are unambiguous, but fail to be @acronym{LALR}(1).
+Such grammars typically require more than one symbol of lookahead,
+or (in rare cases) fall into the category of grammars in which the
+@acronym{LALR}(1) algorithm throws away too much information (they are in
+@acronym{LR}(1), but not @acronym{LALR}(1), @ref{Mystery Conflicts}).
+
+Consider a problem that
+arises in the declaration of enumerated and subrange types in the
+programming language Pascal.  Here are some examples:
+
+@example
+type subrange = lo .. hi;
+type enum = (a, b, c);
+@end example
+
+@noindent
+The original language standard allows only numeric
+literals and constant identifiers for the subrange bounds (@samp{lo}
+and @samp{hi}), but Extended Pascal (@acronym{ISO}/@acronym{IEC}
+10206) and many other
+Pascal implementations allow arbitrary expressions there.  This gives
+rise to the following situation, containing a superfluous pair of
+parentheses:
+
+@example
+type subrange = (a) .. b;
+@end example
+
+@noindent
+Compare this to the following declaration of an enumerated
+type with only one value:
+
+@example
+type enum = (a);
+@end example
+
+@noindent
+(These declarations are contrived, but they are syntactically
+valid, and more-complicated cases can come up in practical programs.)
+
+These two declarations look identical until the @samp{..} token.
+With normal @acronym{LALR}(1) one-token lookahead it is not
+possible to decide between the two forms when the identifier
+@samp{a} is parsed.  It is, however, desirable
+for a parser to decide this, since in the latter case
+@samp{a} must become a new identifier to represent the enumeration
+value, while in the former case @samp{a} must be evaluated with its
+current meaning, which may be a constant or even a function call.
+
+You could parse @samp{(a)} as an ``unspecified identifier in parentheses'',
+to be resolved later, but this typically requires substantial
+contortions in both semantic actions and large parts of the
+grammar, where the parentheses are nested in the recursive rules for
+expressions.
+
+You might think of using the lexer to distinguish between the two
+forms by returning different tokens for currently defined and
+undefined identifiers.  But if these declarations occur in a local
+scope, and @samp{a} is defined in an outer scope, then both forms
+are possible---either locally redefining @samp{a}, or using the
+value of @samp{a} from the outer scope.  So this approach cannot
+work.
+
+A simple solution to this problem is to declare the parser to
+use the @acronym{GLR} algorithm.
+When the @acronym{GLR} parser reaches the critical state, it
+merely splits into two branches and pursues both syntax rules
+simultaneously.  Sooner or later, one of them runs into a parsing
+error.  If there is a @samp{..} token before the next
+@samp{;}, the rule for enumerated types fails since it cannot
+accept @samp{..} anywhere; otherwise, the subrange type rule
+fails since it requires a @samp{..} token.  So one of the branches
+fails silently, and the other one continues normally, performing
+all the intermediate actions that were postponed during the split.
+
+If the input is syntactically incorrect, both branches fail and the parser
+reports a syntax error as usual.
+
+The effect of all this is that the parser seems to ``guess'' the
+correct branch to take, or in other words, it seems to use more
+lookahead than the underlying @acronym{LALR}(1) algorithm actually allows
+for.  In this example, @acronym{LALR}(2) would suffice, but also some cases
+that are not @acronym{LALR}(@math{k}) for any @math{k} can be handled this way.
+
+In general, a @acronym{GLR} parser can take quadratic or cubic worst-case time,
+and the current Bison parser even takes exponential time and space
+for some grammars.  In practice, this rarely happens, and for many
+grammars it is possible to prove that it cannot happen.
+The present example contains only one conflict between two
+rules, and the type-declaration context containing the conflict
+cannot be nested.  So the number of
+branches that can exist at any time is limited by the constant 2,
+and the parsing time is still linear.
+
+Here is a Bison grammar corresponding to the example above.  It
+parses a vastly simplified form of Pascal type declarations.
+
+@example
+%token TYPE DOTDOT ID
+
+@group
+%left '+' '-'
+%left '*' '/'
+@end group
+
+%%
+
+@group
+type_decl : TYPE ID '=' type ';'
+     ;
+@end group
+
+@group
+type : '(' id_list ')'
+     | expr DOTDOT expr
+     ;
+@end group
+
+@group
+id_list : ID
+     | id_list ',' ID
+     ;
+@end group
+
+@group
+expr : '(' expr ')'
+     | expr '+' expr
+     | expr '-' expr
+     | expr '*' expr
+     | expr '/' expr
+     | ID
+     ;
+@end group
+@end example
+
+When used as a normal @acronym{LALR}(1) grammar, Bison correctly complains
+about one reduce/reduce conflict.  In the conflicting situation the
+parser chooses one of the alternatives, arbitrarily the one
+declared first.  Therefore the following correct input is not
+recognized:
+
+@example
+type t = (a) .. b;
+@end example
+
+The parser can be turned into a @acronym{GLR} parser, while also telling Bison
+to be silent about the one known reduce/reduce conflict, by
+adding these two declarations to the Bison input file (before the first
+@samp{%%}):
+
+@example
+%glr-parser
+%expect-rr 1
+@end example
+
+@noindent
+No change in the grammar itself is required.  Now the
+parser recognizes all valid declarations, according to the
+limited syntax above, transparently.  In fact, the user does not even
+notice when the parser splits.
+
+So here we have a case where we can use the benefits of @acronym{GLR},
+almost without disadvantages.  Even in simple cases like this, however,
+there are at least two potential problems to beware.  First, always
+analyze the conflicts reported by Bison to make sure that @acronym{GLR}
+splitting is only done where it is intended.  A @acronym{GLR} parser
+splitting inadvertently may cause problems less obvious than an
+@acronym{LALR} parser statically choosing the wrong alternative in a
+conflict.  Second, consider interactions with the lexer (@pxref{Semantic
+Tokens}) with great care.  Since a split parser consumes tokens without
+performing any actions during the split, the lexer cannot obtain
+information via parser actions.  Some cases of lexer interactions can be
+eliminated by using @acronym{GLR} to shift the complications from the
+lexer to the parser.  You must check the remaining cases for
+correctness.
+
+In our example, it would be safe for the lexer to return tokens based on
+their current meanings in some symbol table, because no new symbols are
+defined in the middle of a type declaration.  Though it is possible for
+a parser to define the enumeration constants as they are parsed, before
+the type declaration is completed, it actually makes no difference since
+they cannot be used within the same enumerated type declaration.
+
+@node Merging GLR Parses
+@subsection Using @acronym{GLR} to Resolve Ambiguities
+@cindex @acronym{GLR} parsing, ambiguous grammars
+@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, ambiguous grammars
+@findex %dprec
+@findex %merge
+@cindex conflicts
+@cindex reduce/reduce conflicts
+
+Let's consider an example, vastly simplified from a C++ grammar.
+
+@example
+%@{
+  #include <stdio.h>
+  #define YYSTYPE char const *
+  int yylex (void);
+  void yyerror (char const *);
+%@}
+
+%token TYPENAME ID
+
+%right '='
+%left '+'
+
+%glr-parser
+
+%%
+
+prog :
+     | prog stmt   @{ printf ("\n"); @}
+     ;
+
+stmt : expr ';'  %dprec 1
+     | decl      %dprec 2
+     ;
+
+expr : ID               @{ printf ("%s ", $$); @}
+     | TYPENAME '(' expr ')'
+                        @{ printf ("%s <cast> ", $1); @}
+     | expr '+' expr    @{ printf ("+ "); @}
+     | expr '=' expr    @{ printf ("= "); @}
+     ;
+
+decl : TYPENAME declarator ';'
+                        @{ printf ("%s <declare> ", $1); @}
+     | TYPENAME declarator '=' expr ';'
+                        @{ printf ("%s <init-declare> ", $1); @}
+     ;
+
+declarator : ID         @{ printf ("\"%s\" ", $1); @}
+     | '(' declarator ')'
+     ;
+@end example
+
+@noindent
+This models a problematic part of the C++ grammar---the ambiguity between
+certain declarations and statements.  For example,
+
+@example
+T (x) = y+z;
+@end example
+
+@noindent
+parses as either an @code{expr} or a @code{stmt}
+(assuming that @samp{T} is recognized as a @code{TYPENAME} and
+@samp{x} as an @code{ID}).
+Bison detects this as a reduce/reduce conflict between the rules
+@code{expr : ID} and @code{declarator : ID}, which it cannot resolve at the
+time it encounters @code{x} in the example above.  Since this is a
+@acronym{GLR} parser, it therefore splits the problem into two parses, one for
+each choice of resolving the reduce/reduce conflict.
+Unlike the example from the previous section (@pxref{Simple GLR Parsers}),
+however, neither of these parses ``dies,'' because the grammar as it stands is
+ambiguous.  One of the parsers eventually reduces @code{stmt : expr ';'} and
+the other reduces @code{stmt : decl}, after which both parsers are in an
+identical state: they've seen @samp{prog stmt} and have the same unprocessed
+input remaining.  We say that these parses have @dfn{merged.}
+
+At this point, the @acronym{GLR} parser requires a specification in the
+grammar of how to choose between the competing parses.
+In the example above, the two @code{%dprec}
+declarations specify that Bison is to give precedence
+to the parse that interprets the example as a
+@code{decl}, which implies that @code{x} is a declarator.
+The parser therefore prints
+
+@example
+"x" y z + T <init-declare>
+@end example
+
+The @code{%dprec} declarations only come into play when more than one
+parse survives.  Consider a different input string for this parser:
+
+@example
+T (x) + y;
+@end example
+
+@noindent
+This is another example of using @acronym{GLR} to parse an unambiguous
+construct, as shown in the previous section (@pxref{Simple GLR Parsers}).
+Here, there is no ambiguity (this cannot be parsed as a declaration).
+However, at the time the Bison parser encounters @code{x}, it does not
+have enough information to resolve the reduce/reduce conflict (again,
+between @code{x} as an @code{expr} or a @code{declarator}).  In this
+case, no precedence declaration is used.  Again, the parser splits
+into two, one assuming that @code{x} is an @code{expr}, and the other
+assuming @code{x} is a @code{declarator}.  The second of these parsers
+then vanishes when it sees @code{+}, and the parser prints
+
+@example
+x T <cast> y +
+@end example
+
+Suppose that instead of resolving the ambiguity, you wanted to see all
+the possibilities.  For this purpose, you must merge the semantic
+actions of the two possible parsers, rather than choosing one over the
+other.  To do so, you could change the declaration of @code{stmt} as
+follows:
+
+@example
+stmt : expr ';'  %merge <stmtMerge>
+     | decl      %merge <stmtMerge>
+     ;
+@end example
+
+@noindent
+and define the @code{stmtMerge} function as:
+
+@example
+static YYSTYPE
+stmtMerge (YYSTYPE x0, YYSTYPE x1)
+@{
+  printf ("<OR> ");
+  return "";
+@}
+@end example
+
+@noindent
+with an accompanying forward declaration
+in the C declarations at the beginning of the file:
+
+@example
+%@{
+  #define YYSTYPE char const *
+  static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);
+%@}
+@end example
+
+@noindent
+With these declarations, the resulting parser parses the first example
+as both an @code{expr} and a @code{decl}, and prints
+
+@example
+"x" y z + T <init-declare> x T <cast> y z + = <OR>
+@end example
+
+Bison requires that all of the
+productions that participate in any particular merge have identical
+@samp{%merge} clauses.  Otherwise, the ambiguity would be unresolvable,
+and the parser will report an error during any parse that results in
+the offending merge.
+
+@node GLR Semantic Actions
+@subsection GLR Semantic Actions
+
+@cindex deferred semantic actions
+By definition, a deferred semantic action is not performed at the same time as
+the associated reduction.
+This raises caveats for several Bison features you might use in a semantic
+action in a @acronym{GLR} parser.
+
+@vindex yychar
+@cindex @acronym{GLR} parsers and @code{yychar}
+@vindex yylval
+@cindex @acronym{GLR} parsers and @code{yylval}
+@vindex yylloc
+@cindex @acronym{GLR} parsers and @code{yylloc}
+In any semantic action, you can examine @code{yychar} to determine the type of
+the lookahead token present at the time of the associated reduction.
+After checking that @code{yychar} is not set to @code{YYEMPTY} or @code{YYEOF},
+you can then examine @code{yylval} and @code{yylloc} to determine the
+lookahead token's semantic value and location, if any.
+In a nondeferred semantic action, you can also modify any of these variables to
+influence syntax analysis.
+@xref{Lookahead, ,Lookahead Tokens}.
+
+@findex yyclearin
+@cindex @acronym{GLR} parsers and @code{yyclearin}
+In a deferred semantic action, it's too late to influence syntax analysis.
+In this case, @code{yychar}, @code{yylval}, and @code{yylloc} are set to
+shallow copies of the values they had at the time of the associated reduction.
+For this reason alone, modifying them is dangerous.
+Moreover, the result of modifying them is undefined and subject to change with
+future versions of Bison.
+For example, if a semantic action might be deferred, you should never write it
+to invoke @code{yyclearin} (@pxref{Action Features}) or to attempt to free
+memory referenced by @code{yylval}.
+
+@findex YYERROR
+@cindex @acronym{GLR} parsers and @code{YYERROR}
+Another Bison feature requiring special consideration is @code{YYERROR}
+(@pxref{Action Features}), which you can invoke in a semantic action to
+initiate error recovery.
+During deterministic @acronym{GLR} operation, the effect of @code{YYERROR} is
+the same as its effect in an @acronym{LALR}(1) parser.
+In a deferred semantic action, its effect is undefined.
+@c The effect is probably a syntax error at the split point.
+
+Also, see @ref{Location Default Action, ,Default Action for Locations}, which
+describes a special usage of @code{YYLLOC_DEFAULT} in @acronym{GLR} parsers.
+
+@node Compiler Requirements
+@subsection Considerations when Compiling @acronym{GLR} Parsers
+@cindex @code{inline}
+@cindex @acronym{GLR} parsers and @code{inline}
+
+The @acronym{GLR} parsers require a compiler for @acronym{ISO} C89 or
+later.  In addition, they use the @code{inline} keyword, which is not
+C89, but is C99 and is a common extension in pre-C99 compilers.  It is
+up to the user of these parsers to handle
+portability issues.  For instance, if using Autoconf and the Autoconf
+macro @code{AC_C_INLINE}, a mere
+
+@example
+%@{
+  #include <config.h>
+%@}
+@end example
+
+@noindent
+will suffice.  Otherwise, we suggest
+
+@example
+%@{
+  #if __STDC_VERSION__ < 199901 && ! defined __GNUC__ && ! defined inline
+   #define inline
+  #endif
+%@}
+@end example
+
+@node Locations Overview
+@section Locations
+@cindex location
+@cindex textual location
+@cindex location, textual
+
+Many applications, like interpreters or compilers, have to produce verbose
+and useful error messages.  To achieve this, one must be able to keep track of
+the @dfn{textual location}, or @dfn{location}, of each syntactic construct.
+Bison provides a mechanism for handling these locations.
+
+Each token has a semantic value.  In a similar fashion, each token has an
+associated location, but the type of locations is the same for all tokens and
+groupings.  Moreover, the output parser is equipped with a default data
+structure for storing locations (@pxref{Locations}, for more details).
+
+Like semantic values, locations can be reached in actions using a dedicated
+set of constructs.  In the example above, the location of the whole grouping
+is @code{@@$}, while the locations of the subexpressions are @code{@@1} and
+@code{@@3}.
+
+When a rule is matched, a default action is used to compute the semantic value
+of its left hand side (@pxref{Actions}).  In the same way, another default
+action is used for locations.  However, the action for locations is general
+enough for most cases, meaning there is usually no need to describe for each
+rule how @code{@@$} should be formed.  When building a new location for a given
+grouping, the default behavior of the output parser is to take the beginning
+of the first symbol, and the end of the last symbol.
+
+@node Bison Parser
+@section Bison Output: the Parser File
+@cindex Bison parser
+@cindex Bison utility
+@cindex lexical analyzer, purpose
+@cindex parser
+
+When you run Bison, you give it a Bison grammar file as input.  The output
+is a C source file that parses the language described by the grammar.
+This file is called a @dfn{Bison parser}.  Keep in mind that the Bison
+utility and the Bison parser are two distinct programs: the Bison utility
+is a program whose output is the Bison parser that becomes part of your
+program.
+
+The job of the Bison parser is to group tokens into groupings according to
+the grammar rules---for example, to build identifiers and operators into
+expressions.  As it does this, it runs the actions for the grammar rules it
+uses.
+
+The tokens come from a function called the @dfn{lexical analyzer} that
+you must supply in some fashion (such as by writing it in C).  The Bison
+parser calls the lexical analyzer each time it wants a new token.  It
+doesn't know what is ``inside'' the tokens (though their semantic values
+may reflect this).  Typically the lexical analyzer makes the tokens by
+parsing characters of text, but Bison does not depend on this.
+@xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+
+The Bison parser file is C code which defines a function named
+@code{yyparse} which implements that grammar.  This function does not make
+a complete C program: you must supply some additional functions.  One is
+the lexical analyzer.  Another is an error-reporting function which the
+parser calls to report an error.  In addition, a complete C program must
+start with a function called @code{main}; you have to provide this, and
+arrange for it to call @code{yyparse} or the parser will never run.
+@xref{Interface, ,Parser C-Language Interface}.
+
+Aside from the token type names and the symbols in the actions you
+write, all symbols defined in the Bison parser file itself
+begin with @samp{yy} or @samp{YY}.  This includes interface functions
+such as the lexical analyzer function @code{yylex}, the error reporting
+function @code{yyerror} and the parser function @code{yyparse} itself.
+This also includes numerous identifiers used for internal purposes.
+Therefore, you should avoid using C identifiers starting with @samp{yy}
+or @samp{YY} in the Bison grammar file except for the ones defined in
+this manual.  Also, you should avoid using the C identifiers
+@samp{malloc} and @samp{free} for anything other than their usual
+meanings.
+
+In some cases the Bison parser file includes system headers, and in
+those cases your code should respect the identifiers reserved by those
+headers.  On some non-@acronym{GNU} hosts, @code{<alloca.h>}, @code{<malloc.h>},
+@code{<stddef.h>}, and @code{<stdlib.h>} are included as needed to
+declare memory allocators and related types.  @code{<libintl.h>} is
+included if message translation is in use
+(@pxref{Internationalization}).  Other system headers may
+be included if you define @code{YYDEBUG} to a nonzero value
+(@pxref{Tracing, ,Tracing Your Parser}).
+
+@node Stages
+@section Stages in Using Bison
+@cindex stages in using Bison
+@cindex using Bison
+
+The actual language-design process using Bison, from grammar specification
+to a working compiler or interpreter, has these parts:
+
+@enumerate
+@item
+Formally specify the grammar in a form recognized by Bison
+(@pxref{Grammar File, ,Bison Grammar Files}).  For each grammatical rule
+in the language, describe the action that is to be taken when an
+instance of that rule is recognized.  The action is described by a
+sequence of C statements.
+
+@item
+Write a lexical analyzer to process input and pass tokens to the parser.
+The lexical analyzer may be written by hand in C (@pxref{Lexical, ,The
+Lexical Analyzer Function @code{yylex}}).  It could also be produced
+using Lex, but the use of Lex is not discussed in this manual.
+
+@item
+Write a controlling function that calls the Bison-produced parser.
+
+@item
+Write error-reporting routines.
+@end enumerate
+
+To turn this source code as written into a runnable program, you
+must follow these steps:
+
+@enumerate
+@item
+Run Bison on the grammar to produce the parser.
+
+@item
+Compile the code output by Bison, as well as any other source files.
+
+@item
+Link the object files to produce the finished product.
+@end enumerate
+
+@node Grammar Layout
+@section The Overall Layout of a Bison Grammar
+@cindex grammar file
+@cindex file format
+@cindex format of grammar file
+@cindex layout of Bison grammar
+
+The input file for the Bison utility is a @dfn{Bison grammar file}.  The
+general form of a Bison grammar file is as follows:
+
+@example
+%@{
+@var{Prologue}
+%@}
+
+@var{Bison declarations}
+
+%%
+@var{Grammar rules}
+%%
+@var{Epilogue}
+@end example
+
+@noindent
+The @samp{%%}, @samp{%@{} and @samp{%@}} are punctuation that appears
+in every Bison grammar file to separate the sections.
+
+The prologue may define types and variables used in the actions.  You can
+also use preprocessor commands to define macros used there, and use
+@code{#include} to include header files that do any of these things.
+You need to declare the lexical analyzer @code{yylex} and the error
+printer @code{yyerror} here, along with any other global identifiers
+used by the actions in the grammar rules.
+
+The Bison declarations declare the names of the terminal and nonterminal
+symbols, and may also describe operator precedence and the data types of
+semantic values of various symbols.
+
+The grammar rules define how to construct each nonterminal symbol from its
+parts.
+
+The epilogue can contain any code you want to use.  Often the
+definitions of functions declared in the prologue go here.  In a
+simple program, all the rest of the program can go here.
+
+@node Examples
+@chapter Examples
+@cindex simple examples
+@cindex examples, simple
+
+Now we show and explain three sample programs written using Bison: a
+reverse polish notation calculator, an algebraic (infix) notation
+calculator, and a multi-function calculator.  All three have been tested
+under BSD Unix 4.3; each produces a usable, though limited, interactive
+desk-top calculator.
+
+These examples are simple, but Bison grammars for real programming
+languages are written the same way.  You can copy these examples into a
+source file to try them.
+
+@menu
+* RPN Calc::               Reverse polish notation calculator;
+                             a first example with no operator precedence.
+* Infix Calc::             Infix (algebraic) notation calculator.
+                             Operator precedence is introduced.
+* Simple Error Recovery::  Continuing after syntax errors.
+* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$.
+* Multi-function Calc::    Calculator with memory and trig functions.
+                             It uses multiple data-types for semantic values.
+* Exercises::              Ideas for improving the multi-function calculator.
+@end menu
+
+@node RPN Calc
+@section Reverse Polish Notation Calculator
+@cindex reverse polish notation
+@cindex polish notation calculator
+@cindex @code{rpcalc}
+@cindex calculator, simple
+
+The first example is that of a simple double-precision @dfn{reverse polish
+notation} calculator (a calculator using postfix operators).  This example
+provides a good starting point, since operator precedence is not an issue.
+The second example will illustrate how operator precedence is handled.
+
+The source code for this calculator is named @file{rpcalc.y}.  The
+@samp{.y} extension is a convention used for Bison input files.
+
+@menu
+* Rpcalc Declarations::    Prologue (declarations) for rpcalc.
+* Rpcalc Rules::           Grammar Rules for rpcalc, with explanation.
+* Rpcalc Lexer::           The lexical analyzer.
+* Rpcalc Main::            The controlling function.
+* Rpcalc Error::           The error reporting function.
+* Rpcalc Generate::        Running Bison on the grammar file.
+* Rpcalc Compile::         Run the C compiler on the output code.
+@end menu
+
+@node Rpcalc Declarations
+@subsection Declarations for @code{rpcalc}
+
+Here are the C and Bison declarations for the reverse polish notation
+calculator.  As in C, comments are placed between @samp{/*@dots{}*/}.
+
+@example
+/* Reverse polish notation calculator.  */
+
+%@{
+  #define YYSTYPE double
+  #include <math.h>
+  int yylex (void);
+  void yyerror (char const *);
+%@}
+
+%token NUM
+
+%% /* Grammar rules and actions follow.  */
+@end example
+
+The declarations section (@pxref{Prologue, , The prologue}) contains two
+preprocessor directives and two forward declarations.
+
+The @code{#define} directive defines the macro @code{YYSTYPE}, thus
+specifying the C data type for semantic values of both tokens and
+groupings (@pxref{Value Type, ,Data Types of Semantic Values}).  The
+Bison parser will use whatever type @code{YYSTYPE} is defined as; if you
+don't define it, @code{int} is the default.  Because we specify
+@code{double}, each token and each expression has an associated value,
+which is a floating point number.
+
+The @code{#include} directive is used to declare the exponentiation
+function @code{pow}.
+
+The forward declarations for @code{yylex} and @code{yyerror} are
+needed because the C language requires that functions be declared
+before they are used.  These functions will be defined in the
+epilogue, but the parser calls them so they must be declared in the
+prologue.
+
+The second section, Bison declarations, provides information to Bison
+about the token types (@pxref{Bison Declarations, ,The Bison
+Declarations Section}).  Each terminal symbol that is not a
+single-character literal must be declared here.  (Single-character
+literals normally don't need to be declared.)  In this example, all the
+arithmetic operators are designated by single-character literals, so the
+only terminal symbol that needs to be declared is @code{NUM}, the token
+type for numeric constants.
+
+@node Rpcalc Rules
+@subsection Grammar Rules for @code{rpcalc}
+
+Here are the grammar rules for the reverse polish notation calculator.
+
+@example
+input:    /* empty */
+        | input line
+;
+
+line:     '\n'
+        | exp '\n'      @{ printf ("\t%.10g\n", $1); @}
+;
+
+exp:      NUM           @{ $$ = $1;           @}
+        | exp exp '+'   @{ $$ = $1 + $2;      @}
+        | exp exp '-'   @{ $$ = $1 - $2;      @}
+        | exp exp '*'   @{ $$ = $1 * $2;      @}
+        | exp exp '/'   @{ $$ = $1 / $2;      @}
+         /* Exponentiation */
+        | exp exp '^'   @{ $$ = pow ($1, $2); @}
+         /* Unary minus    */
+        | exp 'n'       @{ $$ = -$1;          @}
+;
+%%
+@end example
+
+The groupings of the rpcalc ``language'' defined here are the expression
+(given the name @code{exp}), the line of input (@code{line}), and the
+complete input transcript (@code{input}).  Each of these nonterminal
+symbols has several alternate rules, joined by the vertical bar @samp{|}
+which is read as ``or''.  The following sections explain what these rules
+mean.
+
+The semantics of the language is determined by the actions taken when a
+grouping is recognized.  The actions are the C code that appears inside
+braces.  @xref{Actions}.
+
+You must specify these actions in C, but Bison provides the means for
+passing semantic values between the rules.  In each action, the
+pseudo-variable @code{$$} stands for the semantic value for the grouping
+that the rule is going to construct.  Assigning a value to @code{$$} is the
+main job of most actions.  The semantic values of the components of the
+rule are referred to as @code{$1}, @code{$2}, and so on.
+
+@menu
+* Rpcalc Input::
+* Rpcalc Line::
+* Rpcalc Expr::
+@end menu
+
+@node Rpcalc Input
+@subsubsection Explanation of @code{input}
+
+Consider the definition of @code{input}:
+
+@example
+input:    /* empty */
+        | input line
+;
+@end example
+
+This definition reads as follows: ``A complete input is either an empty
+string, or a complete input followed by an input line''.  Notice that
+``complete input'' is defined in terms of itself.  This definition is said
+to be @dfn{left recursive} since @code{input} appears always as the
+leftmost symbol in the sequence.  @xref{Recursion, ,Recursive Rules}.
+
+The first alternative is empty because there are no symbols between the
+colon and the first @samp{|}; this means that @code{input} can match an
+empty string of input (no tokens).  We write the rules this way because it
+is legitimate to type @kbd{Ctrl-d} right after you start the calculator.
+It's conventional to put an empty alternative first and write the comment
+@samp{/* empty */} in it.
+
+The second alternate rule (@code{input line}) handles all nontrivial input.
+It means, ``After reading any number of lines, read one more line if
+possible.''  The left recursion makes this rule into a loop.  Since the
+first alternative matches empty input, the loop can be executed zero or
+more times.
+
+The parser function @code{yyparse} continues to process input until a
+grammatical error is seen or the lexical analyzer says there are no more
+input tokens; we will arrange for the latter to happen at end-of-input.
+
+@node Rpcalc Line
+@subsubsection Explanation of @code{line}
+
+Now consider the definition of @code{line}:
+
+@example
+line:     '\n'
+        | exp '\n'  @{ printf ("\t%.10g\n", $1); @}
+;
+@end example
+
+The first alternative is a token which is a newline character; this means
+that rpcalc accepts a blank line (and ignores it, since there is no
+action).  The second alternative is an expression followed by a newline.
+This is the alternative that makes rpcalc useful.  The semantic value of
+the @code{exp} grouping is the value of @code{$1} because the @code{exp} in
+question is the first symbol in the alternative.  The action prints this
+value, which is the result of the computation the user asked for.
+
+This action is unusual because it does not assign a value to @code{$$}.  As
+a consequence, the semantic value associated with the @code{line} is
+uninitialized (its value will be unpredictable).  This would be a bug if
+that value were ever used, but we don't use it: once rpcalc has printed the
+value of the user's input line, that value is no longer needed.
+
+@node Rpcalc Expr
+@subsubsection Explanation of @code{expr}
+
+The @code{exp} grouping has several rules, one for each kind of expression.
+The first rule handles the simplest expressions: those that are just numbers.
+The second handles an addition-expression, which looks like two expressions
+followed by a plus-sign.  The third handles subtraction, and so on.
+
+@example
+exp:      NUM
+        | exp exp '+'     @{ $$ = $1 + $2;    @}
+        | exp exp '-'     @{ $$ = $1 - $2;    @}
+        @dots{}
+        ;
+@end example
+
+We have used @samp{|} to join all the rules for @code{exp}, but we could
+equally well have written them separately:
+
+@example
+exp:      NUM ;
+exp:      exp exp '+'     @{ $$ = $1 + $2;    @} ;
+exp:      exp exp '-'     @{ $$ = $1 - $2;    @} ;
+        @dots{}
+@end example
+
+Most of the rules have actions that compute the value of the expression in
+terms of the value of its parts.  For example, in the rule for addition,
+@code{$1} refers to the first component @code{exp} and @code{$2} refers to
+the second one.  The third component, @code{'+'}, has no meaningful
+associated semantic value, but if it had one you could refer to it as
+@code{$3}.  When @code{yyparse} recognizes a sum expression using this
+rule, the sum of the two subexpressions' values is produced as the value of
+the entire expression.  @xref{Actions}.
+
+You don't have to give an action for every rule.  When a rule has no
+action, Bison by default copies the value of @code{$1} into @code{$$}.
+This is what happens in the first rule (the one that uses @code{NUM}).
+
+The formatting shown here is the recommended convention, but Bison does
+not require it.  You can add or change white space as much as you wish.
+For example, this:
+
+@example
+exp   : NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{} ;
+@end example
+
+@noindent
+means the same thing as this:
+
+@example
+exp:      NUM
+        | exp exp '+'    @{ $$ = $1 + $2; @}
+        | @dots{}
+;
+@end example
+
+@noindent
+The latter, however, is much more readable.
+
+@node Rpcalc Lexer
+@subsection The @code{rpcalc} Lexical Analyzer
+@cindex writing a lexical analyzer
+@cindex lexical analyzer, writing
+
+The lexical analyzer's job is low-level parsing: converting characters
+or sequences of characters into tokens.  The Bison parser gets its
+tokens by calling the lexical analyzer.  @xref{Lexical, ,The Lexical
+Analyzer Function @code{yylex}}.
+
+Only a simple lexical analyzer is needed for the @acronym{RPN}
+calculator.  This
+lexical analyzer skips blanks and tabs, then reads in numbers as
+@code{double} and returns them as @code{NUM} tokens.  Any other character
+that isn't part of a number is a separate token.  Note that the token-code
+for such a single-character token is the character itself.
+
+The return value of the lexical analyzer function is a numeric code which
+represents a token type.  The same text used in Bison rules to stand for
+this token type is also a C expression for the numeric code for the type.
+This works in two ways.  If the token type is a character literal, then its
+numeric code is that of the character; you can use the same
+character literal in the lexical analyzer to express the number.  If the
+token type is an identifier, that identifier is defined by Bison as a C
+macro whose definition is the appropriate number.  In this example,
+therefore, @code{NUM} becomes a macro for @code{yylex} to use.
+
+The semantic value of the token (if it has one) is stored into the
+global variable @code{yylval}, which is where the Bison parser will look
+for it.  (The C data type of @code{yylval} is @code{YYSTYPE}, which was
+defined at the beginning of the grammar; @pxref{Rpcalc Declarations,
+,Declarations for @code{rpcalc}}.)
+
+A token type code of zero is returned if the end-of-input is encountered.
+(Bison recognizes any nonpositive value as indicating end-of-input.)
+
+Here is the code for the lexical analyzer:
+
+@example
+@group
+/* The lexical analyzer returns a double floating point
+   number on the stack and the token NUM, or the numeric code
+   of the character read if not a number.  It skips all blanks
+   and tabs, and returns 0 for end-of-input.  */
+
+#include <ctype.h>
+@end group
+
+@group
+int
+yylex (void)
+@{
+  int c;
+
+  /* Skip white space.  */
+  while ((c = getchar ()) == ' ' || c == '\t')
+    ;
+@end group
+@group
+  /* Process numbers.  */
+  if (c == '.' || isdigit (c))
+    @{
+      ungetc (c, stdin);
+      scanf ("%lf", &yylval);
+      return NUM;
+    @}
+@end group
+@group
+  /* Return end-of-input.  */
+  if (c == EOF)
+    return 0;
+  /* Return a single char.  */
+  return c;
+@}
+@end group
+@end example
+
+@node Rpcalc Main
+@subsection The Controlling Function
+@cindex controlling function
+@cindex main function in simple example
+
+In keeping with the spirit of this example, the controlling function is
+kept to the bare minimum.  The only requirement is that it call
+@code{yyparse} to start the process of parsing.
+
+@example
+@group
+int
+main (void)
+@{
+  return yyparse ();
+@}
+@end group
+@end example
+
+@node Rpcalc Error
+@subsection The Error Reporting Routine
+@cindex error reporting routine
+
+When @code{yyparse} detects a syntax error, it calls the error reporting
+function @code{yyerror} to print an error message (usually but not
+always @code{"syntax error"}).  It is up to the programmer to supply
+@code{yyerror} (@pxref{Interface, ,Parser C-Language Interface}), so
+here is the definition we will use:
+
+@example
+@group
+#include <stdio.h>
+
+/* Called by yyparse on error.  */
+void
+yyerror (char const *s)
+@{
+  fprintf (stderr, "%s\n", s);
+@}
+@end group
+@end example
+
+After @code{yyerror} returns, the Bison parser may recover from the error
+and continue parsing if the grammar contains a suitable error rule
+(@pxref{Error Recovery}).  Otherwise, @code{yyparse} returns nonzero.  We
+have not written any error rules in this example, so any invalid input will
+cause the calculator program to exit.  This is not clean behavior for a
+real calculator, but it is adequate for the first example.
+
+@node Rpcalc Generate
+@subsection Running Bison to Make the Parser
+@cindex running Bison (introduction)
+
+Before running Bison to produce a parser, we need to decide how to
+arrange all the source code in one or more source files.  For such a
+simple example, the easiest thing is to put everything in one file.  The
+definitions of @code{yylex}, @code{yyerror} and @code{main} go at the
+end, in the epilogue of the file
+(@pxref{Grammar Layout, ,The Overall Layout of a Bison Grammar}).
+
+For a large project, you would probably have several source files, and use
+@code{make} to arrange to recompile them.
+
+With all the source in a single file, you use the following command to
+convert it into a parser file:
+
+@example
+bison @var{file}.y
+@end example
+
+@noindent
+In this example the file was called @file{rpcalc.y} (for ``Reverse Polish
+@sc{calc}ulator'').  Bison produces a file named @file{@var{file}.tab.c},
+removing the @samp{.y} from the original file name.  The file output by
+Bison contains the source code for @code{yyparse}.  The additional
+functions in the input file (@code{yylex}, @code{yyerror} and @code{main})
+are copied verbatim to the output.
+
+@node Rpcalc Compile
+@subsection Compiling the Parser File
+@cindex compiling the parser
+
+Here is how to compile and run the parser file:
+
+@example
+@group
+# @r{List files in current directory.}
+$ @kbd{ls}
+rpcalc.tab.c  rpcalc.y
+@end group
+
+@group
+# @r{Compile the Bison parser.}
+# @r{@samp{-lm} tells compiler to search math library for @code{pow}.}
+$ @kbd{cc -lm -o rpcalc rpcalc.tab.c}
+@end group
+
+@group
+# @r{List files again.}
+$ @kbd{ls}
+rpcalc  rpcalc.tab.c  rpcalc.y
+@end group
+@end example
+
+The file @file{rpcalc} now contains the executable code.  Here is an
+example session using @code{rpcalc}.
+
+@example
+$ @kbd{rpcalc}
+@kbd{4 9 +}
+13
+@kbd{3 7 + 3 4 5 *+-}
+-13
+@kbd{3 7 + 3 4 5 * + - n}              @r{Note the unary minus, @samp{n}}
+13
+@kbd{5 6 / 4 n +}
+-3.166666667
+@kbd{3 4 ^}                            @r{Exponentiation}
+81
+@kbd{^D}                               @r{End-of-file indicator}
+$
+@end example
+
+@node Infix Calc
+@section Infix Notation Calculator: @code{calc}
+@cindex infix notation calculator
+@cindex @code{calc}
+@cindex calculator, infix notation
+
+We now modify rpcalc to handle infix operators instead of postfix.  Infix
+notation involves the concept of operator precedence and the need for
+parentheses nested to arbitrary depth.  Here is the Bison code for
+@file{calc.y}, an infix desk-top calculator.
+
+@example
+/* Infix notation calculator.  */
+
+%@{
+  #define YYSTYPE double
+  #include <math.h>
+  #include <stdio.h>
+  int yylex (void);
+  void yyerror (char const *);
+%@}
+
+/* Bison declarations.  */
+%token NUM
+%left '-' '+'
+%left '*' '/'
+%left NEG     /* negation--unary minus */
+%right '^'    /* exponentiation */
+
+%% /* The grammar follows.  */
+input:    /* empty */
+        | input line
+;
+
+line:     '\n'
+        | exp '\n'  @{ printf ("\t%.10g\n", $1); @}
+;
+
+exp:      NUM                @{ $$ = $1;         @}
+        | exp '+' exp        @{ $$ = $1 + $3;    @}
+        | exp '-' exp        @{ $$ = $1 - $3;    @}
+        | exp '*' exp        @{ $$ = $1 * $3;    @}
+        | exp '/' exp        @{ $$ = $1 / $3;    @}
+        | '-' exp  %prec NEG @{ $$ = -$2;        @}
+        | exp '^' exp        @{ $$ = pow ($1, $3); @}
+        | '(' exp ')'        @{ $$ = $2;         @}
+;
+%%
+@end example
+
+@noindent
+The functions @code{yylex}, @code{yyerror} and @code{main} can be the
+same as before.
+
+There are two important new features shown in this code.
+
+In the second section (Bison declarations), @code{%left} declares token
+types and says they are left-associative operators.  The declarations
+@code{%left} and @code{%right} (right associativity) take the place of
+@code{%token} which is used to declare a token type name without
+associativity.  (These tokens are single-character literals, which
+ordinarily don't need to be declared.  We declare them here to specify
+the associativity.)
+
+Operator precedence is determined by the line ordering of the
+declarations; the higher the line number of the declaration (lower on
+the page or screen), the higher the precedence.  Hence, exponentiation
+has the highest precedence, unary minus (@code{NEG}) is next, followed
+by @samp{*} and @samp{/}, and so on.  @xref{Precedence, ,Operator
+Precedence}.
+
+The other important new feature is the @code{%prec} in the grammar
+section for the unary minus operator.  The @code{%prec} simply instructs
+Bison that the rule @samp{| '-' exp} has the same precedence as
+@code{NEG}---in this case the next-to-highest.  @xref{Contextual
+Precedence, ,Context-Dependent Precedence}.
+
+Here is a sample run of @file{calc.y}:
+
+@need 500
+@example
+$ @kbd{calc}
+@kbd{4 + 4.5 - (34/(8*3+-3))}
+6.880952381
+@kbd{-56 + 2}
+-54
+@kbd{3 ^ 2}
+9
+@end example
+
+@node Simple Error Recovery
+@section Simple Error Recovery
+@cindex error recovery, simple
+
+Up to this point, this manual has not addressed the issue of @dfn{error
+recovery}---how to continue parsing after the parser detects a syntax
+error.  All we have handled is error reporting with @code{yyerror}.
+Recall that by default @code{yyparse} returns after calling
+@code{yyerror}.  This means that an erroneous input line causes the
+calculator program to exit.  Now we show how to rectify this deficiency.
+
+The Bison language itself includes the reserved word @code{error}, which
+may be included in the grammar rules.  In the example below it has
+been added to one of the alternatives for @code{line}:
+
+@example
+@group
+line:     '\n'
+        | exp '\n'   @{ printf ("\t%.10g\n", $1); @}
+        | error '\n' @{ yyerrok;                  @}
+;
+@end group
+@end example
+
+This addition to the grammar allows for simple error recovery in the
+event of a syntax error.  If an expression that cannot be evaluated is
+read, the error will be recognized by the third rule for @code{line},
+and parsing will continue.  (The @code{yyerror} function is still called
+upon to print its message as well.)  The action executes the statement
+@code{yyerrok}, a macro defined automatically by Bison; its meaning is
+that error recovery is complete (@pxref{Error Recovery}).  Note the
+difference between @code{yyerrok} and @code{yyerror}; neither one is a
+misprint.
+
+This form of error recovery deals with syntax errors.  There are other
+kinds of errors; for example, division by zero, which raises an exception
+signal that is normally fatal.  A real calculator program must handle this
+signal and use @code{longjmp} to return to @code{main} and resume parsing
+input lines; it would also have to discard the rest of the current line of
+input.  We won't discuss this issue further because it is not specific to
+Bison programs.
+
+@node Location Tracking Calc
+@section Location Tracking Calculator: @code{ltcalc}
+@cindex location tracking calculator
+@cindex @code{ltcalc}
+@cindex calculator, location tracking
+
+This example extends the infix notation calculator with location
+tracking.  This feature will be used to improve the error messages.  For
+the sake of clarity, this example is a simple integer calculator, since
+most of the work needed to use locations will be done in the lexical
+analyzer.
+
+@menu
+* Ltcalc Declarations::    Bison and C declarations for ltcalc.
+* Ltcalc Rules::           Grammar rules for ltcalc, with explanations.
+* Ltcalc Lexer::           The lexical analyzer.
+@end menu
+
+@node Ltcalc Declarations
+@subsection Declarations for @code{ltcalc}
+
+The C and Bison declarations for the location tracking calculator are
+the same as the declarations for the infix notation calculator.
+
+@example
+/* Location tracking calculator.  */
+
+%@{
+  #define YYSTYPE int
+  #include <math.h>
+  int yylex (void);
+  void yyerror (char const *);
+%@}
+
+/* Bison declarations.  */
+%token NUM
+
+%left '-' '+'
+%left '*' '/'
+%left NEG
+%right '^'
+
+%% /* The grammar follows.  */
+@end example
+
+@noindent
+Note there are no declarations specific to locations.  Defining a data
+type for storing locations is not needed: we will use the type provided
+by default (@pxref{Location Type, ,Data Types of Locations}), which is a
+four member structure with the following integer fields:
+@code{first_line}, @code{first_column}, @code{last_line} and
+@code{last_column}.  By conventions, and in accordance with the GNU
+Coding Standards and common practice, the line and column count both
+start at 1.
+
+@node Ltcalc Rules
+@subsection Grammar Rules for @code{ltcalc}
+
+Whether handling locations or not has no effect on the syntax of your
+language.  Therefore, grammar rules for this example will be very close
+to those of the previous example: we will only modify them to benefit
+from the new information.
+
+Here, we will use locations to report divisions by zero, and locate the
+wrong expressions or subexpressions.
+
+@example
+@group
+input   : /* empty */
+        | input line
+;
+@end group
+
+@group
+line    : '\n'
+        | exp '\n' @{ printf ("%d\n", $1); @}
+;
+@end group
+
+@group
+exp     : NUM           @{ $$ = $1; @}
+        | exp '+' exp   @{ $$ = $1 + $3; @}
+        | exp '-' exp   @{ $$ = $1 - $3; @}
+        | exp '*' exp   @{ $$ = $1 * $3; @}
+@end group
+@group
+        | exp '/' exp
+            @{
+              if ($3)
+                $$ = $1 / $3;
+              else
+                @{
+                  $$ = 1;
+                  fprintf (stderr, "%d.%d-%d.%d: division by zero",
+                           @@3.first_line, @@3.first_column,
+                           @@3.last_line, @@3.last_column);
+                @}
+            @}
+@end group
+@group
+        | '-' exp %prec NEG     @{ $$ = -$2; @}
+        | exp '^' exp           @{ $$ = pow ($1, $3); @}
+        | '(' exp ')'           @{ $$ = $2; @}
+@end group
+@end example
+
+This code shows how to reach locations inside of semantic actions, by
+using the pseudo-variables @code{@@@var{n}} for rule components, and the
+pseudo-variable @code{@@$} for groupings.
+
+We don't need to assign a value to @code{@@$}: the output parser does it
+automatically.  By default, before executing the C code of each action,
+@code{@@$} is set to range from the beginning of @code{@@1} to the end
+of @code{@@@var{n}}, for a rule with @var{n} components.  This behavior
+can be redefined (@pxref{Location Default Action, , Default Action for
+Locations}), and for very specific rules, @code{@@$} can be computed by
+hand.
+
+@node Ltcalc Lexer
+@subsection The @code{ltcalc} Lexical Analyzer.
+
+Until now, we relied on Bison's defaults to enable location
+tracking.  The next step is to rewrite the lexical analyzer, and make it
+able to feed the parser with the token locations, as it already does for
+semantic values.
+
+To this end, we must take into account every single character of the
+input text, to avoid the computed locations of being fuzzy or wrong:
+
+@example
+@group
+int
+yylex (void)
+@{
+  int c;
+@end group
+
+@group
+  /* Skip white space.  */
+  while ((c = getchar ()) == ' ' || c == '\t')
+    ++yylloc.last_column;
+@end group
+
+@group
+  /* Step.  */
+  yylloc.first_line = yylloc.last_line;
+  yylloc.first_column = yylloc.last_column;
+@end group
+
+@group
+  /* Process numbers.  */
+  if (isdigit (c))
+    @{
+      yylval = c - '0';
+      ++yylloc.last_column;
+      while (isdigit (c = getchar ()))
+        @{
+          ++yylloc.last_column;
+          yylval = yylval * 10 + c - '0';
+        @}
+      ungetc (c, stdin);
+      return NUM;
+    @}
+@end group
+
+  /* Return end-of-input.  */
+  if (c == EOF)
+    return 0;
+
+  /* Return a single char, and update location.  */
+  if (c == '\n')
+    @{
+      ++yylloc.last_line;
+      yylloc.last_column = 0;
+    @}
+  else
+    ++yylloc.last_column;
+  return c;
+@}
+@end example
+
+Basically, the lexical analyzer performs the same processing as before:
+it skips blanks and tabs, and reads numbers or single-character tokens.
+In addition, it updates @code{yylloc}, the global variable (of type
+@code{YYLTYPE}) containing the token's location.
+
+Now, each time this function returns a token, the parser has its number
+as well as its semantic value, and its location in the text.  The last
+needed change is to initialize @code{yylloc}, for example in the
+controlling function:
+
+@example
+@group
+int
+main (void)
+@{
+  yylloc.first_line = yylloc.last_line = 1;
+  yylloc.first_column = yylloc.last_column = 0;
+  return yyparse ();
+@}
+@end group
+@end example
+
+Remember that computing locations is not a matter of syntax.  Every
+character must be associated to a location update, whether it is in
+valid input, in comments, in literal strings, and so on.
+
+@node Multi-function Calc
+@section Multi-Function Calculator: @code{mfcalc}
+@cindex multi-function calculator
+@cindex @code{mfcalc}
+@cindex calculator, multi-function
+
+Now that the basics of Bison have been discussed, it is time to move on to
+a more advanced problem.  The above calculators provided only five
+functions, @samp{+}, @samp{-}, @samp{*}, @samp{/} and @samp{^}.  It would
+be nice to have a calculator that provides other mathematical functions such
+as @code{sin}, @code{cos}, etc.
+
+It is easy to add new operators to the infix calculator as long as they are
+only single-character literals.  The lexical analyzer @code{yylex} passes
+back all nonnumeric characters as tokens, so new grammar rules suffice for
+adding a new operator.  But we want something more flexible: built-in
+functions whose syntax has this form:
+
+@example
+@var{function_name} (@var{argument})
+@end example
+
+@noindent
+At the same time, we will add memory to the calculator, by allowing you
+to create named variables, store values in them, and use them later.
+Here is a sample session with the multi-function calculator:
+
+@example
+$ @kbd{mfcalc}
+@kbd{pi = 3.141592653589}
+3.1415926536
+@kbd{sin(pi)}
+0.0000000000
+@kbd{alpha = beta1 = 2.3}
+2.3000000000
+@kbd{alpha}
+2.3000000000
+@kbd{ln(alpha)}
+0.8329091229
+@kbd{exp(ln(beta1))}
+2.3000000000
+$
+@end example
+
+Note that multiple assignment and nested function calls are permitted.
+
+@menu
+* Mfcalc Declarations::    Bison declarations for multi-function calculator.
+* Mfcalc Rules::           Grammar rules for the calculator.
+* Mfcalc Symbol Table::    Symbol table management subroutines.
+@end menu
+
+@node Mfcalc Declarations
+@subsection Declarations for @code{mfcalc}
+
+Here are the C and Bison declarations for the multi-function calculator.
+
+@smallexample
+@group
+%@{
+  #include <math.h>  /* For math functions, cos(), sin(), etc.  */
+  #include "calc.h"  /* Contains definition of `symrec'.  */
+  int yylex (void);
+  void yyerror (char const *);
+%@}
+@end group
+@group
+%union @{
+  double    val;   /* For returning numbers.  */
+  symrec  *tptr;   /* For returning symbol-table pointers.  */
+@}
+@end group
+%token <val>  NUM        /* Simple double precision number.  */
+%token <tptr> VAR FNCT   /* Variable and Function.  */
+%type  <val>  exp
+
+@group
+%right '='
+%left '-' '+'
+%left '*' '/'
+%left NEG     /* negation--unary minus */
+%right '^'    /* exponentiation */
+@end group
+%% /* The grammar follows.  */
+@end smallexample
+
+The above grammar introduces only two new features of the Bison language.
+These features allow semantic values to have various data types
+(@pxref{Multiple Types, ,More Than One Value Type}).
+
+The @code{%union} declaration specifies the entire list of possible types;
+this is instead of defining @code{YYSTYPE}.  The allowable types are now
+double-floats (for @code{exp} and @code{NUM}) and pointers to entries in
+the symbol table.  @xref{Union Decl, ,The Collection of Value Types}.
+
+Since values can now have various types, it is necessary to associate a
+type with each grammar symbol whose semantic value is used.  These symbols
+are @code{NUM}, @code{VAR}, @code{FNCT}, and @code{exp}.  Their
+declarations are augmented with information about their data type (placed
+between angle brackets).
+
+The Bison construct @code{%type} is used for declaring nonterminal
+symbols, just as @code{%token} is used for declaring token types.  We
+have not used @code{%type} before because nonterminal symbols are
+normally declared implicitly by the rules that define them.  But
+@code{exp} must be declared explicitly so we can specify its value type.
+@xref{Type Decl, ,Nonterminal Symbols}.
+
+@node Mfcalc Rules
+@subsection Grammar Rules for @code{mfcalc}
+
+Here are the grammar rules for the multi-function calculator.
+Most of them are copied directly from @code{calc}; three rules,
+those which mention @code{VAR} or @code{FNCT}, are new.
+
+@smallexample
+@group
+input:   /* empty */
+        | input line
+;
+@end group
+
+@group
+line:
+          '\n'
+        | exp '\n'   @{ printf ("\t%.10g\n", $1); @}
+        | error '\n' @{ yyerrok;                  @}
+;
+@end group
+
+@group
+exp:      NUM                @{ $$ = $1;                         @}
+        | VAR                @{ $$ = $1->value.var;              @}
+        | VAR '=' exp        @{ $$ = $3; $1->value.var = $3;     @}
+        | FNCT '(' exp ')'   @{ $$ = (*($1->value.fnctptr))($3); @}
+        | exp '+' exp        @{ $$ = $1 + $3;                    @}
+        | exp '-' exp        @{ $$ = $1 - $3;                    @}
+        | exp '*' exp        @{ $$ = $1 * $3;                    @}
+        | exp '/' exp        @{ $$ = $1 / $3;                    @}
+        | '-' exp  %prec NEG @{ $$ = -$2;                        @}
+        | exp '^' exp        @{ $$ = pow ($1, $3);               @}
+        | '(' exp ')'        @{ $$ = $2;                         @}
+;
+@end group
+/* End of grammar.  */
+%%
+@end smallexample
+
+@node Mfcalc Symbol Table
+@subsection The @code{mfcalc} Symbol Table
+@cindex symbol table example
+
+The multi-function calculator requires a symbol table to keep track of the
+names and meanings of variables and functions.  This doesn't affect the
+grammar rules (except for the actions) or the Bison declarations, but it
+requires some additional C functions for support.
+
+The symbol table itself consists of a linked list of records.  Its
+definition, which is kept in the header @file{calc.h}, is as follows.  It
+provides for either functions or variables to be placed in the table.
+
+@smallexample
+@group
+/* Function type.  */
+typedef double (*func_t) (double);
+@end group
+
+@group
+/* Data type for links in the chain of symbols.  */
+struct symrec
+@{
+  char *name;  /* name of symbol */
+  int type;    /* type of symbol: either VAR or FNCT */
+  union
+  @{
+    double var;      /* value of a VAR */
+    func_t fnctptr;  /* value of a FNCT */
+  @} value;
+  struct symrec *next;  /* link field */
+@};
+@end group
+
+@group
+typedef struct symrec symrec;
+
+/* The symbol table: a chain of `struct symrec'.  */
+extern symrec *sym_table;
+
+symrec *putsym (char const *, int);
+symrec *getsym (char const *);
+@end group
+@end smallexample
+
+The new version of @code{main} includes a call to @code{init_table}, a
+function that initializes the symbol table.  Here it is, and
+@code{init_table} as well:
+
+@smallexample
+#include <stdio.h>
+
+@group
+/* Called by yyparse on error.  */
+void
+yyerror (char const *s)
+@{
+  printf ("%s\n", s);
+@}
+@end group
+
+@group
+struct init
+@{
+  char const *fname;
+  double (*fnct) (double);
+@};
+@end group
+
+@group
+struct init const arith_fncts[] =
+@{
+  "sin",  sin,
+  "cos",  cos,
+  "atan", atan,
+  "ln",   log,
+  "exp",  exp,
+  "sqrt", sqrt,
+  0, 0
+@};
+@end group
+
+@group
+/* The symbol table: a chain of `struct symrec'.  */
+symrec *sym_table;
+@end group
+
+@group
+/* Put arithmetic functions in table.  */
+void
+init_table (void)
+@{
+  int i;
+  symrec *ptr;
+  for (i = 0; arith_fncts[i].fname != 0; i++)
+    @{
+      ptr = putsym (arith_fncts[i].fname, FNCT);
+      ptr->value.fnctptr = arith_fncts[i].fnct;
+    @}
+@}
+@end group
+
+@group
+int
+main (void)
+@{
+  init_table ();
+  return yyparse ();
+@}
+@end group
+@end smallexample
+
+By simply editing the initialization list and adding the necessary include
+files, you can add additional functions to the calculator.
+
+Two important functions allow look-up and installation of symbols in the
+symbol table.  The function @code{putsym} is passed a name and the type
+(@code{VAR} or @code{FNCT}) of the object to be installed.  The object is
+linked to the front of the list, and a pointer to the object is returned.
+The function @code{getsym} is passed the name of the symbol to look up.  If
+found, a pointer to that symbol is returned; otherwise zero is returned.
+
+@smallexample
+symrec *
+putsym (char const *sym_name, int sym_type)
+@{
+  symrec *ptr;
+  ptr = (symrec *) malloc (sizeof (symrec));
+  ptr->name = (char *) malloc (strlen (sym_name) + 1);
+  strcpy (ptr->name,sym_name);
+  ptr->type = sym_type;
+  ptr->value.var = 0; /* Set value to 0 even if fctn.  */
+  ptr->next = (struct symrec *)sym_table;
+  sym_table = ptr;
+  return ptr;
+@}
+
+symrec *
+getsym (char const *sym_name)
+@{
+  symrec *ptr;
+  for (ptr = sym_table; ptr != (symrec *) 0;
+       ptr = (symrec *)ptr->next)
+    if (strcmp (ptr->name,sym_name) == 0)
+      return ptr;
+  return 0;
+@}
+@end smallexample
+
+The function @code{yylex} must now recognize variables, numeric values, and
+the single-character arithmetic operators.  Strings of alphanumeric
+characters with a leading letter are recognized as either variables or
+functions depending on what the symbol table says about them.
+
+The string is passed to @code{getsym} for look up in the symbol table.  If
+the name appears in the table, a pointer to its location and its type
+(@code{VAR} or @code{FNCT}) is returned to @code{yyparse}.  If it is not
+already in the table, then it is installed as a @code{VAR} using
+@code{putsym}.  Again, a pointer and its type (which must be @code{VAR}) is
+returned to @code{yyparse}.
+
+No change is needed in the handling of numeric values and arithmetic
+operators in @code{yylex}.
+
+@smallexample
+@group
+#include <ctype.h>
+@end group
+
+@group
+int
+yylex (void)
+@{
+  int c;
+
+  /* Ignore white space, get first nonwhite character.  */
+  while ((c = getchar ()) == ' ' || c == '\t');
+
+  if (c == EOF)
+    return 0;
+@end group
+
+@group
+  /* Char starts a number => parse the number.         */
+  if (c == '.' || isdigit (c))
+    @{
+      ungetc (c, stdin);
+      scanf ("%lf", &yylval.val);
+      return NUM;
+    @}
+@end group
+
+@group
+  /* Char starts an identifier => read the name.       */
+  if (isalpha (c))
+    @{
+      symrec *s;
+      static char *symbuf = 0;
+      static int length = 0;
+      int i;
+@end group
+
+@group
+      /* Initially make the buffer long enough
+         for a 40-character symbol name.  */
+      if (length == 0)
+        length = 40, symbuf = (char *)malloc (length + 1);
+
+      i = 0;
+      do
+@end group
+@group
+        @{
+          /* If buffer is full, make it bigger.        */
+          if (i == length)
+            @{
+              length *= 2;
+              symbuf = (char *) realloc (symbuf, length + 1);
+            @}
+          /* Add this character to the buffer.         */
+          symbuf[i++] = c;
+          /* Get another character.                    */
+          c = getchar ();
+        @}
+@end group
+@group
+      while (isalnum (c));
+
+      ungetc (c, stdin);
+      symbuf[i] = '\0';
+@end group
+
+@group
+      s = getsym (symbuf);
+      if (s == 0)
+        s = putsym (symbuf, VAR);
+      yylval.tptr = s;
+      return s->type;
+    @}
+
+  /* Any other character is a token by itself.        */
+  return c;
+@}
+@end group
+@end smallexample
+
+This program is both powerful and flexible.  You may easily add new
+functions, and it is a simple job to modify this code to install
+predefined variables such as @code{pi} or @code{e} as well.
+
+@node Exercises
+@section Exercises
+@cindex exercises
+
+@enumerate
+@item
+Add some new functions from @file{math.h} to the initialization list.
+
+@item
+Add another array that contains constants and their values.  Then
+modify @code{init_table} to add these constants to the symbol table.
+It will be easiest to give the constants type @code{VAR}.
+
+@item
+Make the program report an error if the user refers to an
+uninitialized variable in any way except to store a value in it.
+@end enumerate
+
+@node Grammar File
+@chapter Bison Grammar Files
+
+Bison takes as input a context-free grammar specification and produces a
+C-language function that recognizes correct instances of the grammar.
+
+The Bison grammar input file conventionally has a name ending in @samp{.y}.
+@xref{Invocation, ,Invoking Bison}.
+
+@menu
+* Grammar Outline::   Overall layout of the grammar file.
+* Symbols::           Terminal and nonterminal symbols.
+* Rules::             How to write grammar rules.
+* Recursion::         Writing recursive rules.
+* Semantics::         Semantic values and actions.
+* Locations::         Locations and actions.
+* Declarations::      All kinds of Bison declarations are described here.
+* Multiple Parsers::  Putting more than one Bison parser in one program.
+@end menu
+
+@node Grammar Outline
+@section Outline of a Bison Grammar
+
+A Bison grammar file has four main sections, shown here with the
+appropriate delimiters:
+
+@example
+%@{
+  @var{Prologue}
+%@}
+
+@var{Bison declarations}
+
+%%
+@var{Grammar rules}
+%%
+
+@var{Epilogue}
+@end example
+
+Comments enclosed in @samp{/* @dots{} */} may appear in any of the sections.
+As a @acronym{GNU} extension, @samp{//} introduces a comment that
+continues until end of line.
+
+@menu
+* Prologue::              Syntax and usage of the prologue.
+* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
+* Bison Declarations::    Syntax and usage of the Bison declarations section.
+* Grammar Rules::         Syntax and usage of the grammar rules section.
+* Epilogue::              Syntax and usage of the epilogue.
+@end menu
+
+@node Prologue
+@subsection The prologue
+@cindex declarations section
+@cindex Prologue
+@cindex declarations
+
+The @var{Prologue} section contains macro definitions and declarations
+of functions and variables that are used in the actions in the grammar
+rules.  These are copied to the beginning of the parser file so that
+they precede the definition of @code{yyparse}.  You can use
+@samp{#include} to get the declarations from a header file.  If you
+don't need any C declarations, you may omit the @samp{%@{} and
+@samp{%@}} delimiters that bracket this section.
+
+The @var{Prologue} section is terminated by the first occurrence
+of @samp{%@}} that is outside a comment, a string literal, or a
+character constant.
+
+You may have more than one @var{Prologue} section, intermixed with the
+@var{Bison declarations}.  This allows you to have C and Bison
+declarations that refer to each other.  For example, the @code{%union}
+declaration may use types defined in a header file, and you may wish to
+prototype functions that take arguments of type @code{YYSTYPE}.  This
+can be done with two @var{Prologue} blocks, one before and one after the
+@code{%union} declaration.
+
+@smallexample
+%@{
+  #define _GNU_SOURCE
+  #include <stdio.h>
+  #include "ptypes.h"
+%@}
+
+%union @{
+  long int n;
+  tree t;  /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%@{
+  static void print_token_value (FILE *, int, YYSTYPE);
+  #define YYPRINT(F, N, L) print_token_value (F, N, L)
+%@}
+
+@dots{}
+@end smallexample
+
+When in doubt, it is usually safer to put prologue code before all
+Bison declarations, rather than after.  For example, any definitions
+of feature test macros like @code{_GNU_SOURCE} or
+@code{_POSIX_C_SOURCE} should appear before all Bison declarations, as
+feature test macros can affect the behavior of Bison-generated
+@code{#include} directives.
+
+@node Prologue Alternatives
+@subsection Prologue Alternatives
+@cindex Prologue Alternatives
+
+@findex %code
+@findex %code requires
+@findex %code provides
+@findex %code top
+(The prologue alternatives described here are experimental.
+More user feedback will help to determine whether they should become permanent
+features.)
+
+The functionality of @var{Prologue} sections can often be subtle and
+inflexible.
+As an alternative, Bison provides a %code directive with an explicit qualifier
+field, which identifies the purpose of the code and thus the location(s) where
+Bison should generate it.
+For C/C++, the qualifier can be omitted for the default location, or it can be
+one of @code{requires}, @code{provides}, @code{top}.
+@xref{Decl Summary,,%code}.
+
+Look again at the example of the previous section:
+
+@smallexample
+%@{
+  #define _GNU_SOURCE
+  #include <stdio.h>
+  #include "ptypes.h"
+%@}
+
+%union @{
+  long int n;
+  tree t;  /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%@{
+  static void print_token_value (FILE *, int, YYSTYPE);
+  #define YYPRINT(F, N, L) print_token_value (F, N, L)
+%@}
+
+@dots{}
+@end smallexample
+
+@noindent
+Notice that there are two @var{Prologue} sections here, but there's a subtle
+distinction between their functionality.
+For example, if you decide to override Bison's default definition for
+@code{YYLTYPE}, in which @var{Prologue} section should you write your new
+definition?
+You should write it in the first since Bison will insert that code into the
+parser source code file @emph{before} the default @code{YYLTYPE} definition.
+In which @var{Prologue} section should you prototype an internal function,
+@code{trace_token}, that accepts @code{YYLTYPE} and @code{yytokentype} as
+arguments?
+You should prototype it in the second since Bison will insert that code
+@emph{after} the @code{YYLTYPE} and @code{yytokentype} definitions.
+
+This distinction in functionality between the two @var{Prologue} sections is
+established by the appearance of the @code{%union} between them.
+This behavior raises a few questions.
+First, why should the position of a @code{%union} affect definitions related to
+@code{YYLTYPE} and @code{yytokentype}?
+Second, what if there is no @code{%union}?
+In that case, the second kind of @var{Prologue} section is not available.
+This behavior is not intuitive.
+
+To avoid this subtle @code{%union} dependency, rewrite the example using a
+@code{%code top} and an unqualified @code{%code}.
+Let's go ahead and add the new @code{YYLTYPE} definition and the
+@code{trace_token} prototype at the same time:
+
+@smallexample
+%code top @{
+  #define _GNU_SOURCE
+  #include <stdio.h>
+
+  /* WARNING: The following code really belongs
+   * in a `%code requires'; see below.  */
+
+  #include "ptypes.h"
+  #define YYLTYPE YYLTYPE
+  typedef struct YYLTYPE
+  @{
+    int first_line;
+    int first_column;
+    int last_line;
+    int last_column;
+    char *filename;
+  @} YYLTYPE;
+@}
+
+%union @{
+  long int n;
+  tree t;  /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%code @{
+  static void print_token_value (FILE *, int, YYSTYPE);
+  #define YYPRINT(F, N, L) print_token_value (F, N, L)
+  static void trace_token (enum yytokentype token, YYLTYPE loc);
+@}
+
+@dots{}
+@end smallexample
+
+@noindent
+In this way, @code{%code top} and the unqualified @code{%code} achieve the same
+functionality as the two kinds of @var{Prologue} sections, but it's always
+explicit which kind you intend.
+Moreover, both kinds are always available even in the absence of @code{%union}.
+
+The @code{%code top} block above logically contains two parts.
+The first two lines before the warning need to appear near the top of the
+parser source code file.
+The first line after the warning is required by @code{YYSTYPE} and thus also
+needs to appear in the parser source code file.
+However, if you've instructed Bison to generate a parser header file
+(@pxref{Decl Summary, ,%defines}), you probably want that line to appear before
+the @code{YYSTYPE} definition in that header file as well.
+The @code{YYLTYPE} definition should also appear in the parser header file to
+override the default @code{YYLTYPE} definition there.
+
+In other words, in the @code{%code top} block above, all but the first two
+lines are dependency code required by the @code{YYSTYPE} and @code{YYLTYPE}
+definitions.
+Thus, they belong in one or more @code{%code requires}:
+
+@smallexample
+%code top @{
+  #define _GNU_SOURCE
+  #include <stdio.h>
+@}
+
+%code requires @{
+  #include "ptypes.h"
+@}
+%union @{
+  long int n;
+  tree t;  /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%code requires @{
+  #define YYLTYPE YYLTYPE
+  typedef struct YYLTYPE
+  @{
+    int first_line;
+    int first_column;
+    int last_line;
+    int last_column;
+    char *filename;
+  @} YYLTYPE;
+@}
+
+%code @{
+  static void print_token_value (FILE *, int, YYSTYPE);
+  #define YYPRINT(F, N, L) print_token_value (F, N, L)
+  static void trace_token (enum yytokentype token, YYLTYPE loc);
+@}
+
+@dots{}
+@end smallexample
+
+@noindent
+Now Bison will insert @code{#include "ptypes.h"} and the new @code{YYLTYPE}
+definition before the Bison-generated @code{YYSTYPE} and @code{YYLTYPE}
+definitions in both the parser source code file and the parser header file.
+(By the same reasoning, @code{%code requires} would also be the appropriate
+place to write your own definition for @code{YYSTYPE}.)
+
+When you are writing dependency code for @code{YYSTYPE} and @code{YYLTYPE}, you
+should prefer @code{%code requires} over @code{%code top} regardless of whether
+you instruct Bison to generate a parser header file.
+When you are writing code that you need Bison to insert only into the parser
+source code file and that has no special need to appear at the top of that
+file, you should prefer the unqualified @code{%code} over @code{%code top}.
+These practices will make the purpose of each block of your code explicit to
+Bison and to other developers reading your grammar file.
+Following these practices, we expect the unqualified @code{%code} and
+@code{%code requires} to be the most important of the four @var{Prologue}
+alternatives.
+
+At some point while developing your parser, you might decide to provide
+@code{trace_token} to modules that are external to your parser.
+Thus, you might wish for Bison to insert the prototype into both the parser
+header file and the parser source code file.
+Since this function is not a dependency required by @code{YYSTYPE} or
+@code{YYLTYPE}, it doesn't make sense to move its prototype to a
+@code{%code requires}.
+More importantly, since it depends upon @code{YYLTYPE} and @code{yytokentype},
+@code{%code requires} is not sufficient.
+Instead, move its prototype from the unqualified @code{%code} to a
+@code{%code provides}:
+
+@smallexample
+%code top @{
+  #define _GNU_SOURCE
+  #include <stdio.h>
+@}
+
+%code requires @{
+  #include "ptypes.h"
+@}
+%union @{
+  long int n;
+  tree t;  /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%code requires @{
+  #define YYLTYPE YYLTYPE
+  typedef struct YYLTYPE
+  @{
+    int first_line;
+    int first_column;
+    int last_line;
+    int last_column;
+    char *filename;
+  @} YYLTYPE;
+@}
+
+%code provides @{
+  void trace_token (enum yytokentype token, YYLTYPE loc);
+@}
+
+%code @{
+  static void print_token_value (FILE *, int, YYSTYPE);
+  #define YYPRINT(F, N, L) print_token_value (F, N, L)
+@}
+
+@dots{}
+@end smallexample
+
+@noindent
+Bison will insert the @code{trace_token} prototype into both the parser header
+file and the parser source code file after the definitions for
+@code{yytokentype}, @code{YYLTYPE}, and @code{YYSTYPE}.
+
+The above examples are careful to write directives in an order that reflects
+the layout of the generated parser source code and header files:
+@code{%code top}, @code{%code requires}, @code{%code provides}, and then
+@code{%code}.
+While your grammar files may generally be easier to read if you also follow
+this order, Bison does not require it.
+Instead, Bison lets you choose an organization that makes sense to you.
+
+You may declare any of these directives multiple times in the grammar file.
+In that case, Bison concatenates the contained code in declaration order.
+This is the only way in which the position of one of these directives within
+the grammar file affects its functionality.
+
+The result of the previous two properties is greater flexibility in how you may
+organize your grammar file.
+For example, you may organize semantic-type-related directives by semantic
+type:
+
+@smallexample
+%code requires @{ #include "type1.h" @}
+%union @{ type1 field1; @}
+%destructor @{ type1_free ($$); @} <field1>
+%printer @{ type1_print ($$); @} <field1>
+
+%code requires @{ #include "type2.h" @}
+%union @{ type2 field2; @}
+%destructor @{ type2_free ($$); @} <field2>
+%printer @{ type2_print ($$); @} <field2>
+@end smallexample
+
+@noindent
+You could even place each of the above directive groups in the rules section of
+the grammar file next to the set of rules that uses the associated semantic
+type.
+(In the rules section, you must terminate each of those directives with a
+semicolon.)
+And you don't have to worry that some directive (like a @code{%union}) in the
+definitions section is going to adversely affect their functionality in some
+counter-intuitive manner just because it comes first.
+Such an organization is not possible using @var{Prologue} sections.
+
+This section has been concerned with explaining the advantages of the four
+@var{Prologue} alternatives over the original Yacc @var{Prologue}.
+However, in most cases when using these directives, you shouldn't need to
+think about all the low-level ordering issues discussed here.
+Instead, you should simply use these directives to label each block of your
+code according to its purpose and let Bison handle the ordering.
+@code{%code} is the most generic label.
+Move code to @code{%code requires}, @code{%code provides}, or @code{%code top}
+as needed.
+
+@node Bison Declarations
+@subsection The Bison Declarations Section
+@cindex Bison declarations (introduction)
+@cindex declarations, Bison (introduction)
+
+The @var{Bison declarations} section contains declarations that define
+terminal and nonterminal symbols, specify precedence, and so on.
+In some simple grammars you may not need any declarations.
+@xref{Declarations, ,Bison Declarations}.
+
+@node Grammar Rules
+@subsection The Grammar Rules Section
+@cindex grammar rules section
+@cindex rules section for grammar
+
+The @dfn{grammar rules} section contains one or more Bison grammar
+rules, and nothing else.  @xref{Rules, ,Syntax of Grammar Rules}.
+
+There must always be at least one grammar rule, and the first
+@samp{%%} (which precedes the grammar rules) may never be omitted even
+if it is the first thing in the file.
+
+@node Epilogue
+@subsection The epilogue
+@cindex additional C code section
+@cindex epilogue
+@cindex C code, section for additional
+
+The @var{Epilogue} is copied verbatim to the end of the parser file, just as
+the @var{Prologue} is copied to the beginning.  This is the most convenient
+place to put anything that you want to have in the parser file but which need
+not come before the definition of @code{yyparse}.  For example, the
+definitions of @code{yylex} and @code{yyerror} often go here.  Because
+C requires functions to be declared before being used, you often need
+to declare functions like @code{yylex} and @code{yyerror} in the Prologue,
+even if you define them in the Epilogue.
+@xref{Interface, ,Parser C-Language Interface}.
+
+If the last section is empty, you may omit the @samp{%%} that separates it
+from the grammar rules.
+
+The Bison parser itself contains many macros and identifiers whose names
+start with @samp{yy} or @samp{YY}, so it is a good idea to avoid using
+any such names (except those documented in this manual) in the epilogue
+of the grammar file.
+
+@node Symbols
+@section Symbols, Terminal and Nonterminal
+@cindex nonterminal symbol
+@cindex terminal symbol
+@cindex token type
+@cindex symbol
+
+@dfn{Symbols} in Bison grammars represent the grammatical classifications
+of the language.
+
+A @dfn{terminal symbol} (also known as a @dfn{token type}) represents a
+class of syntactically equivalent tokens.  You use the symbol in grammar
+rules to mean that a token in that class is allowed.  The symbol is
+represented in the Bison parser by a numeric code, and the @code{yylex}
+function returns a token type code to indicate what kind of token has
+been read.  You don't need to know what the code value is; you can use
+the symbol to stand for it.
+
+A @dfn{nonterminal symbol} stands for a class of syntactically
+equivalent groupings.  The symbol name is used in writing grammar rules.
+By convention, it should be all lower case.
+
+Symbol names can contain letters, digits (not at the beginning),
+underscores and periods.  Periods make sense only in nonterminals.
+
+There are three ways of writing terminal symbols in the grammar:
+
+@itemize @bullet
+@item
+A @dfn{named token type} is written with an identifier, like an
+identifier in C@.  By convention, it should be all upper case.  Each
+such name must be defined with a Bison declaration such as
+@code{%token}.  @xref{Token Decl, ,Token Type Names}.
+
+@item
+@cindex character token
+@cindex literal token
+@cindex single-character literal
+A @dfn{character token type} (or @dfn{literal character token}) is
+written in the grammar using the same syntax used in C for character
+constants; for example, @code{'+'} is a character token type.  A
+character token type doesn't need to be declared unless you need to
+specify its semantic value data type (@pxref{Value Type, ,Data Types of
+Semantic Values}), associativity, or precedence (@pxref{Precedence,
+,Operator Precedence}).
+
+By convention, a character token type is used only to represent a
+token that consists of that particular character.  Thus, the token
+type @code{'+'} is used to represent the character @samp{+} as a
+token.  Nothing enforces this convention, but if you depart from it,
+your program will confuse other readers.
+
+All the usual escape sequences used in character literals in C can be
+used in Bison as well, but you must not use the null character as a
+character literal because its numeric code, zero, signifies
+end-of-input (@pxref{Calling Convention, ,Calling Convention
+for @code{yylex}}).  Also, unlike standard C, trigraphs have no
+special meaning in Bison character literals, nor is backslash-newline
+allowed.
+
+@item
+@cindex string token
+@cindex literal string token
+@cindex multicharacter literal
+A @dfn{literal string token} is written like a C string constant; for
+example, @code{"<="} is a literal string token.  A literal string token
+doesn't need to be declared unless you need to specify its semantic
+value data type (@pxref{Value Type}), associativity, or precedence
+(@pxref{Precedence}).
+
+You can associate the literal string token with a symbolic name as an
+alias, using the @code{%token} declaration (@pxref{Token Decl, ,Token
+Declarations}).  If you don't do that, the lexical analyzer has to
+retrieve the token number for the literal string token from the
+@code{yytname} table (@pxref{Calling Convention}).
+
+@strong{Warning}: literal string tokens do not work in Yacc.
+
+By convention, a literal string token is used only to represent a token
+that consists of that particular string.  Thus, you should use the token
+type @code{"<="} to represent the string @samp{<=} as a token.  Bison
+does not enforce this convention, but if you depart from it, people who
+read your program will be confused.
+
+All the escape sequences used in string literals in C can be used in
+Bison as well, except that you must not use a null character within a
+string literal.  Also, unlike Standard C, trigraphs have no special
+meaning in Bison string literals, nor is backslash-newline allowed.  A
+literal string token must contain two or more characters; for a token
+containing just one character, use a character token (see above).
+@end itemize
+
+How you choose to write a terminal symbol has no effect on its
+grammatical meaning.  That depends only on where it appears in rules and
+on when the parser function returns that symbol.
+
+The value returned by @code{yylex} is always one of the terminal
+symbols, except that a zero or negative value signifies end-of-input.
+Whichever way you write the token type in the grammar rules, you write
+it the same way in the definition of @code{yylex}.  The numeric code
+for a character token type is simply the positive numeric code of the
+character, so @code{yylex} can use the identical value to generate the
+requisite code, though you may need to convert it to @code{unsigned
+char} to avoid sign-extension on hosts where @code{char} is signed.
+Each named token type becomes a C macro in
+the parser file, so @code{yylex} can use the name to stand for the code.
+(This is why periods don't make sense in terminal symbols.)
+@xref{Calling Convention, ,Calling Convention for @code{yylex}}.
+
+If @code{yylex} is defined in a separate file, you need to arrange for the
+token-type macro definitions to be available there.  Use the @samp{-d}
+option when you run Bison, so that it will write these macro definitions
+into a separate header file @file{@var{name}.tab.h} which you can include
+in the other source files that need it.  @xref{Invocation, ,Invoking Bison}.
+
+If you want to write a grammar that is portable to any Standard C
+host, you must use only nonnull character tokens taken from the basic
+execution character set of Standard C@.  This set consists of the ten
+digits, the 52 lower- and upper-case English letters, and the
+characters in the following C-language string:
+
+@example
+"\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_@{|@}~"
+@end example
+
+The @code{yylex} function and Bison must use a consistent character set
+and encoding for character tokens.  For example, if you run Bison in an
+@acronym{ASCII} environment, but then compile and run the resulting
+program in an environment that uses an incompatible character set like
+@acronym{EBCDIC}, the resulting program may not work because the tables
+generated by Bison will assume @acronym{ASCII} numeric values for
+character tokens.  It is standard practice for software distributions to
+contain C source files that were generated by Bison in an
+@acronym{ASCII} environment, so installers on platforms that are
+incompatible with @acronym{ASCII} must rebuild those files before
+compiling them.
+
+The symbol @code{error} is a terminal symbol reserved for error recovery
+(@pxref{Error Recovery}); you shouldn't use it for any other purpose.
+In particular, @code{yylex} should never return this value.  The default
+value of the error token is 256, unless you explicitly assigned 256 to
+one of your tokens with a @code{%token} declaration.
+
+@node Rules
+@section Syntax of Grammar Rules
+@cindex rule syntax
+@cindex grammar rule syntax
+@cindex syntax of grammar rules
+
+A Bison grammar rule has the following general form:
+
+@example
+@group
+@var{result}: @var{components}@dots{}
+        ;
+@end group
+@end example
+
+@noindent
+where @var{result} is the nonterminal symbol that this rule describes,
+and @var{components} are various terminal and nonterminal symbols that
+are put together by this rule (@pxref{Symbols}).
+
+For example,
+
+@example
+@group
+exp:      exp '+' exp
+        ;
+@end group
+@end example
+
+@noindent
+says that two groupings of type @code{exp}, with a @samp{+} token in between,
+can be combined into a larger grouping of type @code{exp}.
+
+White space in rules is significant only to separate symbols.  You can add
+extra white space as you wish.
+
+Scattered among the components can be @var{actions} that determine
+the semantics of the rule.  An action looks like this:
+
+@example
+@{@var{C statements}@}
+@end example
+
+@noindent
+@cindex braced code
+This is an example of @dfn{braced code}, that is, C code surrounded by
+braces, much like a compound statement in C@.  Braced code can contain
+any sequence of C tokens, so long as its braces are balanced.  Bison
+does not check the braced code for correctness directly; it merely
+copies the code to the output file, where the C compiler can check it.
+
+Within braced code, the balanced-brace count is not affected by braces
+within comments, string literals, or character constants, but it is
+affected by the C digraphs @samp{<%} and @samp{%>} that represent
+braces.  At the top level braced code must be terminated by @samp{@}}
+and not by a digraph.  Bison does not look for trigraphs, so if braced
+code uses trigraphs you should ensure that they do not affect the
+nesting of braces or the boundaries of comments, string literals, or
+character constants.
+
+Usually there is only one action and it follows the components.
+@xref{Actions}.
+
+@findex |
+Multiple rules for the same @var{result} can be written separately or can
+be joined with the vertical-bar character @samp{|} as follows:
+
+@example
+@group
+@var{result}:    @var{rule1-components}@dots{}
+        | @var{rule2-components}@dots{}
+        @dots{}
+        ;
+@end group
+@end example
+
+@noindent
+They are still considered distinct rules even when joined in this way.
+
+If @var{components} in a rule is empty, it means that @var{result} can
+match the empty string.  For example, here is how to define a
+comma-separated sequence of zero or more @code{exp} groupings:
+
+@example
+@group
+expseq:   /* empty */
+        | expseq1
+        ;
+@end group
+
+@group
+expseq1:  exp
+        | expseq1 ',' exp
+        ;
+@end group
+@end example
+
+@noindent
+It is customary to write a comment @samp{/* empty */} in each rule
+with no components.
+
+@node Recursion
+@section Recursive Rules
+@cindex recursive rule
+
+A rule is called @dfn{recursive} when its @var{result} nonterminal
+appears also on its right hand side.  Nearly all Bison grammars need to
+use recursion, because that is the only way to define a sequence of any
+number of a particular thing.  Consider this recursive definition of a
+comma-separated sequence of one or more expressions:
+
+@example
+@group
+expseq1:  exp
+        | expseq1 ',' exp
+        ;
+@end group
+@end example
+
+@cindex left recursion
+@cindex right recursion
+@noindent
+Since the recursive use of @code{expseq1} is the leftmost symbol in the
+right hand side, we call this @dfn{left recursion}.  By contrast, here
+the same construct is defined using @dfn{right recursion}:
+
+@example
+@group
+expseq1:  exp
+        | exp ',' expseq1
+        ;
+@end group
+@end example
+
+@noindent
+Any kind of sequence can be defined using either left recursion or right
+recursion, but you should always use left recursion, because it can
+parse a sequence of any number of elements with bounded stack space.
+Right recursion uses up space on the Bison stack in proportion to the
+number of elements in the sequence, because all the elements must be
+shifted onto the stack before the rule can be applied even once.
+@xref{Algorithm, ,The Bison Parser Algorithm}, for further explanation
+of this.
+
+@cindex mutual recursion
+@dfn{Indirect} or @dfn{mutual} recursion occurs when the result of the
+rule does not appear directly on its right hand side, but does appear
+in rules for other nonterminals which do appear on its right hand
+side.
+
+For example:
+
+@example
+@group
+expr:     primary
+        | primary '+' primary
+        ;
+@end group
+
+@group
+primary:  constant
+        | '(' expr ')'
+        ;
+@end group
+@end example
+
+@noindent
+defines two mutually-recursive nonterminals, since each refers to the
+other.
+
+@node Semantics
+@section Defining Language Semantics
+@cindex defining language semantics
+@cindex language semantics, defining
+
+The grammar rules for a language determine only the syntax.  The semantics
+are determined by the semantic values associated with various tokens and
+groupings, and by the actions taken when various groupings are recognized.
+
+For example, the calculator calculates properly because the value
+associated with each expression is the proper number; it adds properly
+because the action for the grouping @w{@samp{@var{x} + @var{y}}} is to add
+the numbers associated with @var{x} and @var{y}.
+
+@menu
+* Value Type::        Specifying one data type for all semantic values.
+* Multiple Types::    Specifying several alternative data types.
+* Actions::           An action is the semantic definition of a grammar rule.
+* Action Types::      Specifying data types for actions to operate on.
+* Mid-Rule Actions::  Most actions go at the end of a rule.
+                      This says when, why and how to use the exceptional
+                        action in the middle of a rule.
+@end menu
+
+@node Value Type
+@subsection Data Types of Semantic Values
+@cindex semantic value type
+@cindex value type, semantic
+@cindex data types of semantic values
+@cindex default data type
+
+In a simple program it may be sufficient to use the same data type for
+the semantic values of all language constructs.  This was true in the
+@acronym{RPN} and infix calculator examples (@pxref{RPN Calc, ,Reverse Polish
+Notation Calculator}).
+
+Bison normally uses the type @code{int} for semantic values if your
+program uses the same data type for all language constructs.  To
+specify some other type, define @code{YYSTYPE} as a macro, like this:
+
+@example
+#define YYSTYPE double
+@end example
+
+@noindent
+@code{YYSTYPE}'s replacement list should be a type name
+that does not contain parentheses or square brackets.
+This macro definition must go in the prologue of the grammar file
+(@pxref{Grammar Outline, ,Outline of a Bison Grammar}).
+
+@node Multiple Types
+@subsection More Than One Value Type
+
+In most programs, you will need different data types for different kinds
+of tokens and groupings.  For example, a numeric constant may need type
+@code{int} or @code{long int}, while a string constant needs type
+@code{char *}, and an identifier might need a pointer to an entry in the
+symbol table.
+
+To use more than one data type for semantic values in one parser, Bison
+requires you to do two things:
+
+@itemize @bullet
+@item
+Specify the entire collection of possible data types, either by using the
+@code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of
+Value Types}), or by using a @code{typedef} or a @code{#define} to
+define @code{YYSTYPE} to be a union type whose member names are
+the type tags.
+
+@item
+Choose one of those types for each symbol (terminal or nonterminal) for
+which semantic values are used.  This is done for tokens with the
+@code{%token} Bison declaration (@pxref{Token Decl, ,Token Type Names})
+and for groupings with the @code{%type} Bison declaration (@pxref{Type
+Decl, ,Nonterminal Symbols}).
+@end itemize
+
+@node Actions
+@subsection Actions
+@cindex action
+@vindex $$
+@vindex $@var{n}
+
+An action accompanies a syntactic rule and contains C code to be executed
+each time an instance of that rule is recognized.  The task of most actions
+is to compute a semantic value for the grouping built by the rule from the
+semantic values associated with tokens or smaller groupings.
+
+An action consists of braced code containing C statements, and can be
+placed at any position in the rule;
+it is executed at that position.  Most rules have just one action at the
+end of the rule, following all the components.  Actions in the middle of
+a rule are tricky and used only for special purposes (@pxref{Mid-Rule
+Actions, ,Actions in Mid-Rule}).
+
+The C code in an action can refer to the semantic values of the components
+matched by the rule with the construct @code{$@var{n}}, which stands for
+the value of the @var{n}th component.  The semantic value for the grouping
+being constructed is @code{$$}.  Bison translates both of these
+constructs into expressions of the appropriate type when it copies the
+actions into the parser file.  @code{$$} is translated to a modifiable
+lvalue, so it can be assigned to.
+
+Here is a typical example:
+
+@example
+@group
+exp:    @dots{}
+        | exp '+' exp
+            @{ $$ = $1 + $3; @}
+@end group
+@end example
+
+@noindent
+This rule constructs an @code{exp} from two smaller @code{exp} groupings
+connected by a plus-sign token.  In the action, @code{$1} and @code{$3}
+refer to the semantic values of the two component @code{exp} groupings,
+which are the first and third symbols on the right hand side of the rule.
+The sum is stored into @code{$$} so that it becomes the semantic value of
+the addition-expression just recognized by the rule.  If there were a
+useful semantic value associated with the @samp{+} token, it could be
+referred to as @code{$2}.
+
+Note that the vertical-bar character @samp{|} is really a rule
+separator, and actions are attached to a single rule.  This is a
+difference with tools like Flex, for which @samp{|} stands for either
+``or'', or ``the same action as that of the next rule''.  In the
+following example, the action is triggered only when @samp{b} is found:
+
+@example
+@group
+a-or-b: 'a'|'b'   @{ a_or_b_found = 1; @};
+@end group
+@end example
+
+@cindex default action
+If you don't specify an action for a rule, Bison supplies a default:
+@w{@code{$$ = $1}.}  Thus, the value of the first symbol in the rule
+becomes the value of the whole rule.  Of course, the default action is
+valid only if the two data types match.  There is no meaningful default
+action for an empty rule; every empty rule must have an explicit action
+unless the rule's value does not matter.
+
+@code{$@var{n}} with @var{n} zero or negative is allowed for reference
+to tokens and groupings on the stack @emph{before} those that match the
+current rule.  This is a very risky practice, and to use it reliably
+you must be certain of the context in which the rule is applied.  Here
+is a case in which you can use this reliably:
+
+@example
+@group
+foo:      expr bar '+' expr  @{ @dots{} @}
+        | expr bar '-' expr  @{ @dots{} @}
+        ;
+@end group
+
+@group
+bar:      /* empty */
+        @{ previous_expr = $0; @}
+        ;
+@end group
+@end example
+
+As long as @code{bar} is used only in the fashion shown here, @code{$0}
+always refers to the @code{expr} which precedes @code{bar} in the
+definition of @code{foo}.
+
+@vindex yylval
+It is also possible to access the semantic value of the lookahead token, if
+any, from a semantic action.
+This semantic value is stored in @code{yylval}.
+@xref{Action Features, ,Special Features for Use in Actions}.
+
+@node Action Types
+@subsection Data Types of Values in Actions
+@cindex action data types
+@cindex data types in actions
+
+If you have chosen a single data type for semantic values, the @code{$$}
+and @code{$@var{n}} constructs always have that data type.
+
+If you have used @code{%union} to specify a variety of data types, then you
+must declare a choice among these types for each terminal or nonterminal
+symbol that can have a semantic value.  Then each time you use @code{$$} or
+@code{$@var{n}}, its data type is determined by which symbol it refers to
+in the rule.  In this example,
+
+@example
+@group
+exp:    @dots{}
+        | exp '+' exp
+            @{ $$ = $1 + $3; @}
+@end group
+@end example
+
+@noindent
+@code{$1} and @code{$3} refer to instances of @code{exp}, so they all
+have the data type declared for the nonterminal symbol @code{exp}.  If
+@code{$2} were used, it would have the data type declared for the
+terminal symbol @code{'+'}, whatever that might be.
+
+Alternatively, you can specify the data type when you refer to the value,
+by inserting @samp{<@var{type}>} after the @samp{$} at the beginning of the
+reference.  For example, if you have defined types as shown here:
+
+@example
+@group
+%union @{
+  int itype;
+  double dtype;
+@}
+@end group
+@end example
+
+@noindent
+then you can write @code{$<itype>1} to refer to the first subunit of the
+rule as an integer, or @code{$<dtype>1} to refer to it as a double.
+
+@node Mid-Rule Actions
+@subsection Actions in Mid-Rule
+@cindex actions in mid-rule
+@cindex mid-rule actions
+
+Occasionally it is useful to put an action in the middle of a rule.
+These actions are written just like usual end-of-rule actions, but they
+are executed before the parser even recognizes the following components.
+
+A mid-rule action may refer to the components preceding it using
+@code{$@var{n}}, but it may not refer to subsequent components because
+it is run before they are parsed.
+
+The mid-rule action itself counts as one of the components of the rule.
+This makes a difference when there is another action later in the same rule
+(and usually there is another at the end): you have to count the actions
+along with the symbols when working out which number @var{n} to use in
+@code{$@var{n}}.
+
+The mid-rule action can also have a semantic value.  The action can set
+its value with an assignment to @code{$$}, and actions later in the rule
+can refer to the value using @code{$@var{n}}.  Since there is no symbol
+to name the action, there is no way to declare a data type for the value
+in advance, so you must use the @samp{$<@dots{}>@var{n}} construct to
+specify a data type each time you refer to this value.
+
+There is no way to set the value of the entire rule with a mid-rule
+action, because assignments to @code{$$} do not have that effect.  The
+only way to set the value for the entire rule is with an ordinary action
+at the end of the rule.
+
+Here is an example from a hypothetical compiler, handling a @code{let}
+statement that looks like @samp{let (@var{variable}) @var{statement}} and
+serves to create a variable named @var{variable} temporarily for the
+duration of @var{statement}.  To parse this construct, we must put
+@var{variable} into the symbol table while @var{statement} is parsed, then
+remove it afterward.  Here is how it is done:
+
+@example
+@group
+stmt:   LET '(' var ')'
+                @{ $<context>$ = push_context ();
+                  declare_variable ($3); @}
+        stmt    @{ $$ = $6;
+                  pop_context ($<context>5); @}
+@end group
+@end example
+
+@noindent
+As soon as @samp{let (@var{variable})} has been recognized, the first
+action is run.  It saves a copy of the current semantic context (the
+list of accessible variables) as its semantic value, using alternative
+@code{context} in the data-type union.  Then it calls
+@code{declare_variable} to add the new variable to that list.  Once the
+first action is finished, the embedded statement @code{stmt} can be
+parsed.  Note that the mid-rule action is component number 5, so the
+@samp{stmt} is component number 6.
+
+After the embedded statement is parsed, its semantic value becomes the
+value of the entire @code{let}-statement.  Then the semantic value from the
+earlier action is used to restore the prior list of variables.  This
+removes the temporary @code{let}-variable from the list so that it won't
+appear to exist while the rest of the program is parsed.
+
+@findex %destructor
+@cindex discarded symbols, mid-rule actions
+@cindex error recovery, mid-rule actions
+In the above example, if the parser initiates error recovery (@pxref{Error
+Recovery}) while parsing the tokens in the embedded statement @code{stmt},
+it might discard the previous semantic context @code{$<context>5} without
+restoring it.
+Thus, @code{$<context>5} needs a destructor (@pxref{Destructor Decl, , Freeing
+Discarded Symbols}).
+However, Bison currently provides no means to declare a destructor specific to
+a particular mid-rule action's semantic value.
+
+One solution is to bury the mid-rule action inside a nonterminal symbol and to
+declare a destructor for that symbol:
+
+@example
+@group
+%type <context> let
+%destructor @{ pop_context ($$); @} let
+
+%%
+
+stmt:  let stmt
+               @{ $$ = $2;
+                 pop_context ($1); @}
+       ;
+
+let:   LET '(' var ')'
+               @{ $$ = push_context ();
+                 declare_variable ($3); @}
+       ;
+
+@end group
+@end example
+
+@noindent
+Note that the action is now at the end of its rule.
+Any mid-rule action can be converted to an end-of-rule action in this way, and
+this is what Bison actually does to implement mid-rule actions.
+
+Taking action before a rule is completely recognized often leads to
+conflicts since the parser must commit to a parse in order to execute the
+action.  For example, the following two rules, without mid-rule actions,
+can coexist in a working parser because the parser can shift the open-brace
+token and look at what follows before deciding whether there is a
+declaration or not:
+
+@example
+@group
+compound: '@{' declarations statements '@}'
+        | '@{' statements '@}'
+        ;
+@end group
+@end example
+
+@noindent
+But when we add a mid-rule action as follows, the rules become nonfunctional:
+
+@example
+@group
+compound: @{ prepare_for_local_variables (); @}
+          '@{' declarations statements '@}'
+@end group
+@group
+        | '@{' statements '@}'
+        ;
+@end group
+@end example
+
+@noindent
+Now the parser is forced to decide whether to run the mid-rule action
+when it has read no farther than the open-brace.  In other words, it
+must commit to using one rule or the other, without sufficient
+information to do it correctly.  (The open-brace token is what is called
+the @dfn{lookahead} token at this time, since the parser is still
+deciding what to do about it.  @xref{Lookahead, ,Lookahead Tokens}.)
+
+You might think that you could correct the problem by putting identical
+actions into the two rules, like this:
+
+@example
+@group
+compound: @{ prepare_for_local_variables (); @}
+          '@{' declarations statements '@}'
+        | @{ prepare_for_local_variables (); @}
+          '@{' statements '@}'
+        ;
+@end group
+@end example
+
+@noindent
+But this does not help, because Bison does not realize that the two actions
+are identical.  (Bison never tries to understand the C code in an action.)
+
+If the grammar is such that a declaration can be distinguished from a
+statement by the first token (which is true in C), then one solution which
+does work is to put the action after the open-brace, like this:
+
+@example
+@group
+compound: '@{' @{ prepare_for_local_variables (); @}
+          declarations statements '@}'
+        | '@{' statements '@}'
+        ;
+@end group
+@end example
+
+@noindent
+Now the first token of the following declaration or statement,
+which would in any case tell Bison which rule to use, can still do so.
+
+Another solution is to bury the action inside a nonterminal symbol which
+serves as a subroutine:
+
+@example
+@group
+subroutine: /* empty */
+          @{ prepare_for_local_variables (); @}
+        ;
+
+@end group
+
+@group
+compound: subroutine
+          '@{' declarations statements '@}'
+        | subroutine
+          '@{' statements '@}'
+        ;
+@end group
+@end example
+
+@noindent
+Now Bison can execute the action in the rule for @code{subroutine} without
+deciding which rule for @code{compound} it will eventually use.
+
+@node Locations
+@section Tracking Locations
+@cindex location
+@cindex textual location
+@cindex location, textual
+
+Though grammar rules and semantic actions are enough to write a fully
+functional parser, it can be useful to process some additional information,
+especially symbol locations.
+
+The way locations are handled is defined by providing a data type, and
+actions to take when rules are matched.
+
+@menu
+* Location Type::               Specifying a data type for locations.
+* Actions and Locations::       Using locations in actions.
+* Location Default Action::     Defining a general way to compute locations.
+@end menu
+
+@node Location Type
+@subsection Data Type of Locations
+@cindex data type of locations
+@cindex default location type
+
+Defining a data type for locations is much simpler than for semantic values,
+since all tokens and groupings always use the same type.
+
+You can specify the type of locations by defining a macro called
+@code{YYLTYPE}, just as you can specify the semantic value type by
+defining a @code{YYSTYPE} macro (@pxref{Value Type}).
+When @code{YYLTYPE} is not defined, Bison uses a default structure type with
+four members:
+
+@example
+typedef struct YYLTYPE
+@{
+  int first_line;
+  int first_column;
+  int last_line;
+  int last_column;
+@} YYLTYPE;
+@end example
+
+At the beginning of the parsing, Bison initializes all these fields to 1
+for @code{yylloc}.
+
+@node Actions and Locations
+@subsection Actions and Locations
+@cindex location actions
+@cindex actions, location
+@vindex @@$
+@vindex @@@var{n}
+
+Actions are not only useful for defining language semantics, but also for
+describing the behavior of the output parser with locations.
+
+The most obvious way for building locations of syntactic groupings is very
+similar to the way semantic values are computed.  In a given rule, several
+constructs can be used to access the locations of the elements being matched.
+The location of the @var{n}th component of the right hand side is
+@code{@@@var{n}}, while the location of the left hand side grouping is
+@code{@@$}.
+
+Here is a basic example using the default data type for locations:
+
+@example
+@group
+exp:    @dots{}
+        | exp '/' exp
+            @{
+              @@$.first_column = @@1.first_column;
+              @@$.first_line = @@1.first_line;
+              @@$.last_column = @@3.last_column;
+              @@$.last_line = @@3.last_line;
+              if ($3)
+                $$ = $1 / $3;
+              else
+                @{
+                  $$ = 1;
+                  fprintf (stderr,
+                           "Division by zero, l%d,c%d-l%d,c%d",
+                           @@3.first_line, @@3.first_column,
+                           @@3.last_line, @@3.last_column);
+                @}
+            @}
+@end group
+@end example
+
+As for semantic values, there is a default action for locations that is
+run each time a rule is matched.  It sets the beginning of @code{@@$} to the
+beginning of the first symbol, and the end of @code{@@$} to the end of the
+last symbol.
+
+With this default action, the location tracking can be fully automatic.  The
+example above simply rewrites this way:
+
+@example
+@group
+exp:    @dots{}
+        | exp '/' exp
+            @{
+              if ($3)
+                $$ = $1 / $3;
+              else
+                @{
+                  $$ = 1;
+                  fprintf (stderr,
+                           "Division by zero, l%d,c%d-l%d,c%d",
+                           @@3.first_line, @@3.first_column,
+                           @@3.last_line, @@3.last_column);
+                @}
+            @}
+@end group
+@end example
+
+@vindex yylloc
+It is also possible to access the location of the lookahead token, if any,
+from a semantic action.
+This location is stored in @code{yylloc}.
+@xref{Action Features, ,Special Features for Use in Actions}.
+
+@node Location Default Action
+@subsection Default Action for Locations
+@vindex YYLLOC_DEFAULT
+@cindex @acronym{GLR} parsers and @code{YYLLOC_DEFAULT}
+
+Actually, actions are not the best place to compute locations.  Since
+locations are much more general than semantic values, there is room in
+the output parser to redefine the default action to take for each
+rule.  The @code{YYLLOC_DEFAULT} macro is invoked each time a rule is
+matched, before the associated action is run.  It is also invoked
+while processing a syntax error, to compute the error's location.
+Before reporting an unresolvable syntactic ambiguity, a @acronym{GLR}
+parser invokes @code{YYLLOC_DEFAULT} recursively to compute the location
+of that ambiguity.
+
+Most of the time, this macro is general enough to suppress location
+dedicated code from semantic actions.
+
+The @code{YYLLOC_DEFAULT} macro takes three parameters.  The first one is
+the location of the grouping (the result of the computation).  When a
+rule is matched, the second parameter identifies locations of
+all right hand side elements of the rule being matched, and the third
+parameter is the size of the rule's right hand side.
+When a @acronym{GLR} parser reports an ambiguity, which of multiple candidate
+right hand sides it passes to @code{YYLLOC_DEFAULT} is undefined.
+When processing a syntax error, the second parameter identifies locations
+of the symbols that were discarded during error processing, and the third
+parameter is the number of discarded symbols.
+
+By default, @code{YYLLOC_DEFAULT} is defined this way:
+
+@smallexample
+@group
+# define YYLLOC_DEFAULT(Current, Rhs, N)                                \
+    do                                                                  \
+      if (N)                                                            \
+        @{                                                               \
+          (Current).first_line   = YYRHSLOC(Rhs, 1).first_line;         \
+          (Current).first_column = YYRHSLOC(Rhs, 1).first_column;       \
+          (Current).last_line    = YYRHSLOC(Rhs, N).last_line;          \
+          (Current).last_column  = YYRHSLOC(Rhs, N).last_column;        \
+        @}                                                               \
+      else                                                              \
+        @{                                                               \
+          (Current).first_line   = (Current).last_line   =              \
+            YYRHSLOC(Rhs, 0).last_line;                                 \
+          (Current).first_column = (Current).last_column =              \
+            YYRHSLOC(Rhs, 0).last_column;                               \
+        @}                                                               \
+    while (0)
+@end group
+@end smallexample
+
+where @code{YYRHSLOC (rhs, k)} is the location of the @var{k}th symbol
+in @var{rhs} when @var{k} is positive, and the location of the symbol
+just before the reduction when @var{k} and @var{n} are both zero.
+
+When defining @code{YYLLOC_DEFAULT}, you should consider that:
+
+@itemize @bullet
+@item
+All arguments are free of side-effects.  However, only the first one (the
+result) should be modified by @code{YYLLOC_DEFAULT}.
+
+@item
+For consistency with semantic actions, valid indexes within the
+right hand side range from 1 to @var{n}.  When @var{n} is zero, only 0 is a
+valid index, and it refers to the symbol just before the reduction.
+During error processing @var{n} is always positive.
+
+@item
+Your macro should parenthesize its arguments, if need be, since the
+actual arguments may not be surrounded by parentheses.  Also, your
+macro should expand to something that can be used as a single
+statement when it is followed by a semicolon.
+@end itemize
+
+@node Declarations
+@section Bison Declarations
+@cindex declarations, Bison
+@cindex Bison declarations
+
+The @dfn{Bison declarations} section of a Bison grammar defines the symbols
+used in formulating the grammar and the data types of semantic values.
+@xref{Symbols}.
+
+All token type names (but not single-character literal tokens such as
+@code{'+'} and @code{'*'}) must be declared.  Nonterminal symbols must be
+declared if you need to specify which data type to use for the semantic
+value (@pxref{Multiple Types, ,More Than One Value Type}).
+
+The first rule in the file also specifies the start symbol, by default.
+If you want some other symbol to be the start symbol, you must declare
+it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free
+Grammars}).
+
+@menu
+* Require Decl::      Requiring a Bison version.
+* Token Decl::        Declaring terminal symbols.
+* Precedence Decl::   Declaring terminals with precedence and associativity.
+* Union Decl::        Declaring the set of all semantic value types.
+* Type Decl::         Declaring the choice of type for a nonterminal symbol.
+* Initial Action Decl::  Code run before parsing starts.
+* Destructor Decl::   Declaring how symbols are freed.
+* Expect Decl::       Suppressing warnings about parsing conflicts.
+* Start Decl::        Specifying the start symbol.
+* Pure Decl::         Requesting a reentrant parser.
+* Push Decl::         Requesting a push parser.
+* Decl Summary::      Table of all Bison declarations.
+@end menu
+
+@node Require Decl
+@subsection Require a Version of Bison
+@cindex version requirement
+@cindex requiring a version of Bison
+@findex %require
+
+You may require the minimum version of Bison to process the grammar.  If
+the requirement is not met, @command{bison} exits with an error (exit
+status 63).
+
+@example
+%require "@var{version}"
+@end example
+
+@node Token Decl
+@subsection Token Type Names
+@cindex declaring token type names
+@cindex token type names, declaring
+@cindex declaring literal string tokens
+@findex %token
+
+The basic way to declare a token type name (terminal symbol) is as follows:
+
+@example
+%token @var{name}
+@end example
+
+Bison will convert this into a @code{#define} directive in
+the parser, so that the function @code{yylex} (if it is in this file)
+can use the name @var{name} to stand for this token type's code.
+
+Alternatively, you can use @code{%left}, @code{%right}, or
+@code{%nonassoc} instead of @code{%token}, if you wish to specify
+associativity and precedence.  @xref{Precedence Decl, ,Operator
+Precedence}.
+
+You can explicitly specify the numeric code for a token type by appending
+a nonnegative decimal or hexadecimal integer value in the field immediately
+following the token name:
+
+@example
+%token NUM 300
+%token XNUM 0x12d // a GNU extension
+@end example
+
+@noindent
+It is generally best, however, to let Bison choose the numeric codes for
+all token types.  Bison will automatically select codes that don't conflict
+with each other or with normal characters.
+
+In the event that the stack type is a union, you must augment the
+@code{%token} or other token declaration to include the data type
+alternative delimited by angle-brackets (@pxref{Multiple Types, ,More
+Than One Value Type}).
+
+For example:
+
+@example
+@group
+%union @{              /* define stack type */
+  double val;
+  symrec *tptr;
+@}
+%token <val> NUM      /* define token NUM and its type */
+@end group
+@end example
+
+You can associate a literal string token with a token type name by
+writing the literal string at the end of a @code{%token}
+declaration which declares the name.  For example:
+
+@example
+%token arrow "=>"
+@end example
+
+@noindent
+For example, a grammar for the C language might specify these names with
+equivalent literal string tokens:
+
+@example
+%token  <operator>  OR      "||"
+%token  <operator>  LE 134  "<="
+%left  OR  "<="
+@end example
+
+@noindent
+Once you equate the literal string and the token name, you can use them
+interchangeably in further declarations or the grammar rules.  The
+@code{yylex} function can use the token name or the literal string to
+obtain the token type code number (@pxref{Calling Convention}).
+Syntax error messages passed to @code{yyerror} from the parser will reference
+the literal string instead of the token name.
+
+The token numbered as 0 corresponds to end of file; the following line
+allows for nicer error messages referring to ``end of file'' instead
+of ``$end'':
+
+@example
+%token END 0 "end of file"
+@end example
+
+@node Precedence Decl
+@subsection Operator Precedence
+@cindex precedence declarations
+@cindex declaring operator precedence
+@cindex operator precedence, declaring
+
+Use the @code{%left}, @code{%right} or @code{%nonassoc} declaration to
+declare a token and specify its precedence and associativity, all at
+once.  These are called @dfn{precedence declarations}.
+@xref{Precedence, ,Operator Precedence}, for general information on
+operator precedence.
+
+The syntax of a precedence declaration is nearly the same as that of
+@code{%token}: either
+
+@example
+%left @var{symbols}@dots{}
+@end example
+
+@noindent
+or
+
+@example
+%left <@var{type}> @var{symbols}@dots{}
+@end example
+
+And indeed any of these declarations serves the purposes of @code{%token}.
+But in addition, they specify the associativity and relative precedence for
+all the @var{symbols}:
+
+@itemize @bullet
+@item
+The associativity of an operator @var{op} determines how repeated uses
+of the operator nest: whether @samp{@var{x} @var{op} @var{y} @var{op}
+@var{z}} is parsed by grouping @var{x} with @var{y} first or by
+grouping @var{y} with @var{z} first.  @code{%left} specifies
+left-associativity (grouping @var{x} with @var{y} first) and
+@code{%right} specifies right-associativity (grouping @var{y} with
+@var{z} first).  @code{%nonassoc} specifies no associativity, which
+means that @samp{@var{x} @var{op} @var{y} @var{op} @var{z}} is
+considered a syntax error.
+
+@item
+The precedence of an operator determines how it nests with other operators.
+All the tokens declared in a single precedence declaration have equal
+precedence and nest together according to their associativity.
+When two tokens declared in different precedence declarations associate,
+the one declared later has the higher precedence and is grouped first.
+@end itemize
+
+For backward compatibility, there is a confusing difference between the
+argument lists of @code{%token} and precedence declarations.
+Only a @code{%token} can associate a literal string with a token type name.
+A precedence declaration always interprets a literal string as a reference to a
+separate token.
+For example:
+
+@example
+%left  OR "<="         // Does not declare an alias.
+%left  OR 134 "<=" 135 // Declares 134 for OR and 135 for "<=".
+@end example
+
+@node Union Decl
+@subsection The Collection of Value Types
+@cindex declaring value types
+@cindex value types, declaring
+@findex %union
+
+The @code{%union} declaration specifies the entire collection of
+possible data types for semantic values.  The keyword @code{%union} is
+followed by braced code containing the same thing that goes inside a
+@code{union} in C@.
+
+For example:
+
+@example
+@group
+%union @{
+  double val;
+  symrec *tptr;
+@}
+@end group
+@end example
+
+@noindent
+This says that the two alternative types are @code{double} and @code{symrec
+*}.  They are given names @code{val} and @code{tptr}; these names are used
+in the @code{%token} and @code{%type} declarations to pick one of the types
+for a terminal or nonterminal symbol (@pxref{Type Decl, ,Nonterminal Symbols}).
+
+As an extension to @acronym{POSIX}, a tag is allowed after the
+@code{union}.  For example:
+
+@example
+@group
+%union value @{
+  double val;
+  symrec *tptr;
+@}
+@end group
+@end example
+
+@noindent
+specifies the union tag @code{value}, so the corresponding C type is
+@code{union value}.  If you do not specify a tag, it defaults to
+@code{YYSTYPE}.
+
+As another extension to @acronym{POSIX}, you may specify multiple
+@code{%union} declarations; their contents are concatenated.  However,
+only the first @code{%union} declaration can specify a tag.
+
+Note that, unlike making a @code{union} declaration in C, you need not write
+a semicolon after the closing brace.
+
+Instead of @code{%union}, you can define and use your own union type
+@code{YYSTYPE} if your grammar contains at least one
+@samp{<@var{type}>} tag.  For example, you can put the following into
+a header file @file{parser.h}:
+
+@example
+@group
+union YYSTYPE @{
+  double val;
+  symrec *tptr;
+@};
+typedef union YYSTYPE YYSTYPE;
+@end group
+@end example
+
+@noindent
+and then your grammar can use the following
+instead of @code{%union}:
+
+@example
+@group
+%@{
+#include "parser.h"
+%@}
+%type <val> expr
+%token <tptr> ID
+@end group
+@end example
+
+@node Type Decl
+@subsection Nonterminal Symbols
+@cindex declaring value types, nonterminals
+@cindex value types, nonterminals, declaring
+@findex %type
+
+@noindent
+When you use @code{%union} to specify multiple value types, you must
+declare the value type of each nonterminal symbol for which values are
+used.  This is done with a @code{%type} declaration, like this:
+
+@example
+%type <@var{type}> @var{nonterminal}@dots{}
+@end example
+
+@noindent
+Here @var{nonterminal} is the name of a nonterminal symbol, and
+@var{type} is the name given in the @code{%union} to the alternative
+that you want (@pxref{Union Decl, ,The Collection of Value Types}).  You
+can give any number of nonterminal symbols in the same @code{%type}
+declaration, if they have the same value type.  Use spaces to separate
+the symbol names.
+
+You can also declare the value type of a terminal symbol.  To do this,
+use the same @code{<@var{type}>} construction in a declaration for the
+terminal symbol.  All kinds of token declarations allow
+@code{<@var{type}>}.
+
+@node Initial Action Decl
+@subsection Performing Actions before Parsing
+@findex %initial-action
+
+Sometimes your parser needs to perform some initializations before
+parsing.  The @code{%initial-action} directive allows for such arbitrary
+code.
+
+@deffn {Directive} %initial-action @{ @var{code} @}
+@findex %initial-action
+Declare that the braced @var{code} must be invoked before parsing each time
+@code{yyparse} is called.  The @var{code} may use @code{$$} and
+@code{@@$} --- initial value and location of the lookahead --- and the
+@code{%parse-param}.
+@end deffn
+
+For instance, if your locations use a file name, you may use
+
+@example
+%parse-param @{ char const *file_name @};
+%initial-action
+@{
+  @@$.initialize (file_name);
+@};
+@end example
+
+
+@node Destructor Decl
+@subsection Freeing Discarded Symbols
+@cindex freeing discarded symbols
+@findex %destructor
+@findex <*>
+@findex <>
+During error recovery (@pxref{Error Recovery}), symbols already pushed
+on the stack and tokens coming from the rest of the file are discarded
+until the parser falls on its feet.  If the parser runs out of memory,
+or if it returns via @code{YYABORT} or @code{YYACCEPT}, all the
+symbols on the stack must be discarded.  Even if the parser succeeds, it
+must discard the start symbol.
+
+When discarded symbols convey heap based information, this memory is
+lost.  While this behavior can be tolerable for batch parsers, such as
+in traditional compilers, it is unacceptable for programs like shells or
+protocol implementations that may parse and execute indefinitely.
+
+The @code{%destructor} directive defines code that is called when a
+symbol is automatically discarded.
+
+@deffn {Directive} %destructor @{ @var{code} @} @var{symbols}
+@findex %destructor
+Invoke the braced @var{code} whenever the parser discards one of the
+@var{symbols}.
+Within @var{code}, @code{$$} designates the semantic value associated
+with the discarded symbol, and @code{@@$} designates its location.
+The additional parser parameters are also available (@pxref{Parser Function, ,
+The Parser Function @code{yyparse}}).
+
+When a symbol is listed among @var{symbols}, its @code{%destructor} is called a
+per-symbol @code{%destructor}.
+You may also define a per-type @code{%destructor} by listing a semantic type
+tag among @var{symbols}.
+In that case, the parser will invoke this @var{code} whenever it discards any
+grammar symbol that has that semantic type tag unless that symbol has its own
+per-symbol @code{%destructor}.
+
+Finally, you can define two different kinds of default @code{%destructor}s.
+(These default forms are experimental.
+More user feedback will help to determine whether they should become permanent
+features.)
+You can place each of @code{<*>} and @code{<>} in the @var{symbols} list of
+exactly one @code{%destructor} declaration in your grammar file.
+The parser will invoke the @var{code} associated with one of these whenever it
+discards any user-defined grammar symbol that has no per-symbol and no per-type
+@code{%destructor}.
+The parser uses the @var{code} for @code{<*>} in the case of such a grammar
+symbol for which you have formally declared a semantic type tag (@code{%type}
+counts as such a declaration, but @code{$<tag>$} does not).
+The parser uses the @var{code} for @code{<>} in the case of such a grammar
+symbol that has no declared semantic type tag.
+@end deffn
+
+@noindent
+For example:
+
+@smallexample
+%union @{ char *string; @}
+%token <string> STRING1
+%token <string> STRING2
+%type  <string> string1
+%type  <string> string2
+%union @{ char character; @}
+%token <character> CHR
+%type  <character> chr
+%token TAGLESS
+
+%destructor @{ @} <character>
+%destructor @{ free ($$); @} <*>
+%destructor @{ free ($$); printf ("%d", @@$.first_line); @} STRING1 string1
+%destructor @{ printf ("Discarding tagless symbol.\n"); @} <>
+@end smallexample
+
+@noindent
+guarantees that, when the parser discards any user-defined symbol that has a
+semantic type tag other than @code{<character>}, it passes its semantic value
+to @code{free} by default.
+However, when the parser discards a @code{STRING1} or a @code{string1}, it also
+prints its line number to @code{stdout}.
+It performs only the second @code{%destructor} in this case, so it invokes
+@code{free} only once.
+Finally, the parser merely prints a message whenever it discards any symbol,
+such as @code{TAGLESS}, that has no semantic type tag.
+
+A Bison-generated parser invokes the default @code{%destructor}s only for
+user-defined as opposed to Bison-defined symbols.
+For example, the parser will not invoke either kind of default
+@code{%destructor} for the special Bison-defined symbols @code{$accept},
+@code{$undefined}, or @code{$end} (@pxref{Table of Symbols, ,Bison Symbols}),
+none of which you can reference in your grammar.
+It also will not invoke either for the @code{error} token (@pxref{Table of
+Symbols, ,error}), which is always defined by Bison regardless of whether you
+reference it in your grammar.
+However, it may invoke one of them for the end token (token 0) if you
+redefine it from @code{$end} to, for example, @code{END}:
+
+@smallexample
+%token END 0
+@end smallexample
+
+@cindex actions in mid-rule
+@cindex mid-rule actions
+Finally, Bison will never invoke a @code{%destructor} for an unreferenced
+mid-rule semantic value (@pxref{Mid-Rule Actions,,Actions in Mid-Rule}).
+That is, Bison does not consider a mid-rule to have a semantic value if you do
+not reference @code{$$} in the mid-rule's action or @code{$@var{n}} (where
+@var{n} is the RHS symbol position of the mid-rule) in any later action in that
+rule.
+However, if you do reference either, the Bison-generated parser will invoke the
+@code{<>} @code{%destructor} whenever it discards the mid-rule symbol.
+
+@ignore
+@noindent
+In the future, it may be possible to redefine the @code{error} token as a
+nonterminal that captures the discarded symbols.
+In that case, the parser will invoke the default destructor for it as well.
+@end ignore
+
+@sp 1
+
+@cindex discarded symbols
+@dfn{Discarded symbols} are the following:
+
+@itemize
+@item
+stacked symbols popped during the first phase of error recovery,
+@item
+incoming terminals during the second phase of error recovery,
+@item
+the current lookahead and the entire stack (except the current
+right-hand side symbols) when the parser returns immediately, and
+@item
+the start symbol, when the parser succeeds.
+@end itemize
+
+The parser can @dfn{return immediately} because of an explicit call to
+@code{YYABORT} or @code{YYACCEPT}, or failed error recovery, or memory
+exhaustion.
+
+Right-hand side symbols of a rule that explicitly triggers a syntax
+error via @code{YYERROR} are not discarded automatically.  As a rule
+of thumb, destructors are invoked only when user actions cannot manage
+the memory.
+
+@node Expect Decl
+@subsection Suppressing Conflict Warnings
+@cindex suppressing conflict warnings
+@cindex preventing warnings about conflicts
+@cindex warnings, preventing
+@cindex conflicts, suppressing warnings of
+@findex %expect
+@findex %expect-rr
+
+Bison normally warns if there are any conflicts in the grammar
+(@pxref{Shift/Reduce, ,Shift/Reduce Conflicts}), but most real grammars
+have harmless shift/reduce conflicts which are resolved in a predictable
+way and would be difficult to eliminate.  It is desirable to suppress
+the warning about these conflicts unless the number of conflicts
+changes.  You can do this with the @code{%expect} declaration.
+
+The declaration looks like this:
+
+@example
+%expect @var{n}
+@end example
+
+Here @var{n} is a decimal integer.  The declaration says there should
+be @var{n} shift/reduce conflicts and no reduce/reduce conflicts.
+Bison reports an error if the number of shift/reduce conflicts differs
+from @var{n}, or if there are any reduce/reduce conflicts.
+
+For normal @acronym{LALR}(1) parsers, reduce/reduce conflicts are more
+serious, and should be eliminated entirely.  Bison will always report
+reduce/reduce conflicts for these parsers.  With @acronym{GLR}
+parsers, however, both kinds of conflicts are routine; otherwise,
+there would be no need to use @acronym{GLR} parsing.  Therefore, it is
+also possible to specify an expected number of reduce/reduce conflicts
+in @acronym{GLR} parsers, using the declaration:
+
+@example
+%expect-rr @var{n}
+@end example
+
+In general, using @code{%expect} involves these steps:
+
+@itemize @bullet
+@item
+Compile your grammar without @code{%expect}.  Use the @samp{-v} option
+to get a verbose list of where the conflicts occur.  Bison will also
+print the number of conflicts.
+
+@item
+Check each of the conflicts to make sure that Bison's default
+resolution is what you really want.  If not, rewrite the grammar and
+go back to the beginning.
+
+@item
+Add an @code{%expect} declaration, copying the number @var{n} from the
+number which Bison printed.  With @acronym{GLR} parsers, add an
+@code{%expect-rr} declaration as well.
+@end itemize
+
+Now Bison will warn you if you introduce an unexpected conflict, but
+will keep silent otherwise.
+
+@node Start Decl
+@subsection The Start-Symbol
+@cindex declaring the start symbol
+@cindex start symbol, declaring
+@cindex default start symbol
+@findex %start
+
+Bison assumes by default that the start symbol for the grammar is the first
+nonterminal specified in the grammar specification section.  The programmer
+may override this restriction with the @code{%start} declaration as follows:
+
+@example
+%start @var{symbol}
+@end example
+
+@node Pure Decl
+@subsection A Pure (Reentrant) Parser
+@cindex reentrant parser
+@cindex pure parser
+@findex %define api.pure
+
+A @dfn{reentrant} program is one which does not alter in the course of
+execution; in other words, it consists entirely of @dfn{pure} (read-only)
+code.  Reentrancy is important whenever asynchronous execution is possible;
+for example, a nonreentrant program may not be safe to call from a signal
+handler.  In systems with multiple threads of control, a nonreentrant
+program must be called only within interlocks.
+
+Normally, Bison generates a parser which is not reentrant.  This is
+suitable for most uses, and it permits compatibility with Yacc.  (The
+standard Yacc interfaces are inherently nonreentrant, because they use
+statically allocated variables for communication with @code{yylex},
+including @code{yylval} and @code{yylloc}.)
+
+Alternatively, you can generate a pure, reentrant parser.  The Bison
+declaration @code{%define api.pure} says that you want the parser to be
+reentrant.  It looks like this:
+
+@example
+%define api.pure
+@end example
+
+The result is that the communication variables @code{yylval} and
+@code{yylloc} become local variables in @code{yyparse}, and a different
+calling convention is used for the lexical analyzer function
+@code{yylex}.  @xref{Pure Calling, ,Calling Conventions for Pure
+Parsers}, for the details of this.  The variable @code{yynerrs}
+becomes local in @code{yyparse} in pull mode but it becomes a member
+of yypstate in push mode.  (@pxref{Error Reporting, ,The Error
+Reporting Function @code{yyerror}}).  The convention for calling
+@code{yyparse} itself is unchanged.
+
+Whether the parser is pure has nothing to do with the grammar rules.
+You can generate either a pure parser or a nonreentrant parser from any
+valid grammar.
+
+@node Push Decl
+@subsection A Push Parser
+@cindex push parser
+@cindex push parser
+@findex %define api.push_pull
+
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+
+A pull parser is called once and it takes control until all its input
+is completely parsed.  A push parser, on the other hand, is called
+each time a new token is made available.
+
+A push parser is typically useful when the parser is part of a
+main event loop in the client's application.  This is typically
+a requirement of a GUI, when the main event loop needs to be triggered
+within a certain time period.
+
+Normally, Bison generates a pull parser.
+The following Bison declaration says that you want the parser to be a push
+parser (@pxref{Decl Summary,,%define api.push_pull}):
+
+@example
+%define api.push_pull "push"
+@end example
+
+In almost all cases, you want to ensure that your push parser is also
+a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}).  The only
+time you should create an impure push parser is to have backwards
+compatibility with the impure Yacc pull mode interface.  Unless you know
+what you are doing, your declarations should look like this:
+
+@example
+%define api.pure
+%define api.push_pull "push"
+@end example
+
+There is a major notable functional difference between the pure push parser
+and the impure push parser.  It is acceptable for a pure push parser to have
+many parser instances, of the same type of parser, in memory at the same time.
+An impure push parser should only use one parser at a time.
+
+When a push parser is selected, Bison will generate some new symbols in
+the generated parser.  @code{yypstate} is a structure that the generated
+parser uses to store the parser's state.  @code{yypstate_new} is the
+function that will create a new parser instance.  @code{yypstate_delete}
+will free the resources associated with the corresponding parser instance.
+Finally, @code{yypush_parse} is the function that should be called whenever a
+token is available to provide the parser.  A trivial example
+of using a pure push parser would look like this:
+
+@example
+int status;
+yypstate *ps = yypstate_new ();
+do @{
+  status = yypush_parse (ps, yylex (), NULL);
+@} while (status == YYPUSH_MORE);
+yypstate_delete (ps);
+@end example
+
+If the user decided to use an impure push parser, a few things about
+the generated parser will change.  The @code{yychar} variable becomes
+a global variable instead of a variable in the @code{yypush_parse} function.
+For this reason, the signature of the @code{yypush_parse} function is
+changed to remove the token as a parameter.  A nonreentrant push parser
+example would thus look like this:
+
+@example
+extern int yychar;
+int status;
+yypstate *ps = yypstate_new ();
+do @{
+  yychar = yylex ();
+  status = yypush_parse (ps);
+@} while (status == YYPUSH_MORE);
+yypstate_delete (ps);
+@end example
+
+That's it. Notice the next token is put into the global variable @code{yychar}
+for use by the next invocation of the @code{yypush_parse} function.
+
+Bison also supports both the push parser interface along with the pull parser
+interface in the same generated parser.  In order to get this functionality,
+you should replace the @code{%define api.push_pull "push"} declaration with the
+@code{%define api.push_pull "both"} declaration.  Doing this will create all of
+the symbols mentioned earlier along with the two extra symbols, @code{yyparse}
+and @code{yypull_parse}.  @code{yyparse} can be used exactly as it normally
+would be used.  However, the user should note that it is implemented in the
+generated parser by calling @code{yypull_parse}.
+This makes the @code{yyparse} function that is generated with the
+@code{%define api.push_pull "both"} declaration slower than the normal
+@code{yyparse} function.  If the user
+calls the @code{yypull_parse} function it will parse the rest of the input
+stream.  It is possible to @code{yypush_parse} tokens to select a subgrammar
+and then @code{yypull_parse} the rest of the input stream.  If you would like
+to switch back and forth between between parsing styles, you would have to
+write your own @code{yypull_parse} function that knows when to quit looking
+for input.  An example of using the @code{yypull_parse} function would look
+like this:
+
+@example
+yypstate *ps = yypstate_new ();
+yypull_parse (ps); /* Will call the lexer */
+yypstate_delete (ps);
+@end example
+
+Adding the @code{%define api.pure} declaration does exactly the same thing to
+the generated parser with @code{%define api.push_pull "both"} as it did for
+@code{%define api.push_pull "push"}.
+
+@node Decl Summary
+@subsection Bison Declaration Summary
+@cindex Bison declaration summary
+@cindex declaration summary
+@cindex summary, Bison declaration
+
+Here is a summary of the declarations used to define a grammar:
+
+@deffn {Directive} %union
+Declare the collection of data types that semantic values may have
+(@pxref{Union Decl, ,The Collection of Value Types}).
+@end deffn
+
+@deffn {Directive} %token
+Declare a terminal symbol (token type name) with no precedence
+or associativity specified (@pxref{Token Decl, ,Token Type Names}).
+@end deffn
+
+@deffn {Directive} %right
+Declare a terminal symbol (token type name) that is right-associative
+(@pxref{Precedence Decl, ,Operator Precedence}).
+@end deffn
+
+@deffn {Directive} %left
+Declare a terminal symbol (token type name) that is left-associative
+(@pxref{Precedence Decl, ,Operator Precedence}).
+@end deffn
+
+@deffn {Directive} %nonassoc
+Declare a terminal symbol (token type name) that is nonassociative
+(@pxref{Precedence Decl, ,Operator Precedence}).
+Using it in a way that would be associative is a syntax error.
+@end deffn
+
+@ifset defaultprec
+@deffn {Directive} %default-prec
+Assign a precedence to rules lacking an explicit @code{%prec} modifier
+(@pxref{Contextual Precedence, ,Context-Dependent Precedence}).
+@end deffn
+@end ifset
+
+@deffn {Directive} %type
+Declare the type of semantic values for a nonterminal symbol
+(@pxref{Type Decl, ,Nonterminal Symbols}).
+@end deffn
+
+@deffn {Directive} %start
+Specify the grammar's start symbol (@pxref{Start Decl, ,The
+Start-Symbol}).
+@end deffn
+
+@deffn {Directive} %expect
+Declare the expected number of shift-reduce conflicts
+(@pxref{Expect Decl, ,Suppressing Conflict Warnings}).
+@end deffn
+
+
+@sp 1
+@noindent
+In order to change the behavior of @command{bison}, use the following
+directives:
+
+@deffn {Directive} %code @{@var{code}@}
+@findex %code
+This is the unqualified form of the @code{%code} directive.
+It inserts @var{code} verbatim at a language-dependent default location in the
+output@footnote{The default location is actually skeleton-dependent;
+  writers of non-standard skeletons however should choose the default location
+  consistently with the behavior of the standard Bison skeletons.}.
+
+@cindex Prologue
+For C/C++, the default location is the parser source code
+file after the usual contents of the parser header file.
+Thus, @code{%code} replaces the traditional Yacc prologue,
+@code{%@{@var{code}%@}}, for most purposes.
+For a detailed discussion, see @ref{Prologue Alternatives}.
+
+For Java, the default location is inside the parser class.
+
+(Like all the Yacc prologue alternatives, this directive is experimental.
+More user feedback will help to determine whether it should become a permanent
+feature.)
+@end deffn
+
+@deffn {Directive} %code @var{qualifier} @{@var{code}@}
+This is the qualified form of the @code{%code} directive.
+If you need to specify location-sensitive verbatim @var{code} that does not
+belong at the default location selected by the unqualified @code{%code} form,
+use this form instead.
+
+@var{qualifier} identifies the purpose of @var{code} and thus the location(s)
+where Bison should generate it.
+Not all values of @var{qualifier} are available for all target languages:
+
+@itemize @bullet
+@item requires
+@findex %code requires
+
+@itemize @bullet
+@item Language(s): C, C++
+
+@item Purpose: This is the best place to write dependency code required for
+@code{YYSTYPE} and @code{YYLTYPE}.
+In other words, it's the best place to define types referenced in @code{%union}
+directives, and it's the best place to override Bison's default @code{YYSTYPE}
+and @code{YYLTYPE} definitions.
+
+@item Location(s): The parser header file and the parser source code file
+before the Bison-generated @code{YYSTYPE} and @code{YYLTYPE} definitions.
+@end itemize
+
+@item provides
+@findex %code provides
+
+@itemize @bullet
+@item Language(s): C, C++
+
+@item Purpose: This is the best place to write additional definitions and
+declarations that should be provided to other modules.
+
+@item Location(s): The parser header file and the parser source code file after
+the Bison-generated @code{YYSTYPE}, @code{YYLTYPE}, and token definitions.
+@end itemize
+
+@item top
+@findex %code top
+
+@itemize @bullet
+@item Language(s): C, C++
+
+@item Purpose: The unqualified @code{%code} or @code{%code requires} should
+usually be more appropriate than @code{%code top}.
+However, occasionally it is necessary to insert code much nearer the top of the
+parser source code file.
+For example:
+
+@smallexample
+%code top @{
+  #define _GNU_SOURCE
+  #include <stdio.h>
+@}
+@end smallexample
+
+@item Location(s): Near the top of the parser source code file.
+@end itemize
+
+@item imports
+@findex %code imports
+
+@itemize @bullet
+@item Language(s): Java
+
+@item Purpose: This is the best place to write Java import directives.
+
+@item Location(s): The parser Java file after any Java package directive and
+before any class definitions.
+@end itemize
+@end itemize
+
+(Like all the Yacc prologue alternatives, this directive is experimental.
+More user feedback will help to determine whether it should become a permanent
+feature.)
+
+@cindex Prologue
+For a detailed discussion of how to use @code{%code} in place of the
+traditional Yacc prologue for C/C++, see @ref{Prologue Alternatives}.
+@end deffn
+
+@deffn {Directive} %debug
+In the parser file, define the macro @code{YYDEBUG} to 1 if it is not
+already defined, so that the debugging facilities are compiled.
+@end deffn
+@xref{Tracing, ,Tracing Your Parser}.
+
+@deffn {Directive} %define @var{variable}
+@deffnx {Directive} %define @var{variable} "@var{value}"
+Define a variable to adjust Bison's behavior.
+The possible choices for @var{variable}, as well as their meanings, depend on
+the selected target language and/or the parser skeleton (@pxref{Decl
+Summary,,%language}, @pxref{Decl Summary,,%skeleton}).
+
+Bison will warn if a @var{variable} is defined multiple times.
+
+Omitting @code{"@var{value}"} is always equivalent to specifying it as
+@code{""}.
+
+Some @var{variable}s may be used as Booleans.
+In this case, Bison will complain if the variable definition does not meet one
+of the following four conditions:
+
+@enumerate
+@item @code{"@var{value}"} is @code{"true"}
+
+@item @code{"@var{value}"} is omitted (or is @code{""}).
+This is equivalent to @code{"true"}.
+
+@item @code{"@var{value}"} is @code{"false"}.
+
+@item @var{variable} is never defined.
+In this case, Bison selects a default value, which may depend on the selected
+target language and/or parser skeleton.
+@end enumerate
+
+Some of the accepted @var{variable}s are:
+
+@itemize @bullet
+@item api.pure
+@findex %define api.pure
+
+@itemize @bullet
+@item Language(s): C
+
+@item Purpose: Request a pure (reentrant) parser program.
+@xref{Pure Decl, ,A Pure (Reentrant) Parser}.
+
+@item Accepted Values: Boolean
+
+@item Default Value: @code{"false"}
+@end itemize
+
+@item api.push_pull
+@findex %define api.push_pull
+
+@itemize @bullet
+@item Language(s): C (LALR(1) only)
+
+@item Purpose: Requests a pull parser, a push parser, or both.
+@xref{Push Decl, ,A Push Parser}.
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+
+@item Accepted Values: @code{"pull"}, @code{"push"}, @code{"both"}
+
+@item Default Value: @code{"pull"}
+@end itemize
+
+@item lr.keep_unreachable_states
+@findex %define lr.keep_unreachable_states
+
+@itemize @bullet
+@item Language(s): all
+
+@item Purpose: Requests that Bison allow unreachable parser states to remain in
+the parser tables.
+Bison considers a state to be unreachable if there exists no sequence of
+transitions from the start state to that state.
+A state can become unreachable during conflict resolution if Bison disables a
+shift action leading to it from a predecessor state.
+Keeping unreachable states is sometimes useful for analysis purposes, but they
+are useless in the generated parser.
+
+@item Accepted Values: Boolean
+
+@item Default Value: @code{"false"}
+
+@item Caveats:
+
+@itemize @bullet
+
+@item Unreachable states may contain conflicts and may use rules not used in
+any other state.
+Thus, keeping unreachable states may induce warnings that are irrelevant to
+your parser's behavior, and it may eliminate warnings that are relevant.
+Of course, the change in warnings may actually be relevant to a parser table
+analysis that wants to keep unreachable states, so this behavior will likely
+remain in future Bison releases.
+
+@item While Bison is able to remove unreachable states, it is not guaranteed to
+remove other kinds of useless states.
+Specifically, when Bison disables reduce actions during conflict resolution,
+some goto actions may become useless, and thus some additional states may
+become useless.
+If Bison were to compute which goto actions were useless and then disable those
+actions, it could identify such states as unreachable and then remove those
+states.
+However, Bison does not compute which goto actions are useless.
+@end itemize
+@end itemize
+
+@item namespace
+@findex %define namespace
+
+@itemize
+@item Languages(s): C++
+
+@item Purpose: Specifies the namespace for the parser class.
+For example, if you specify:
+
+@smallexample
+%define namespace "foo::bar"
+@end smallexample
+
+Bison uses @code{foo::bar} verbatim in references such as:
+
+@smallexample
+foo::bar::parser::semantic_type
+@end smallexample
+
+However, to open a namespace, Bison removes any leading @code{::} and then
+splits on any remaining occurrences:
+
+@smallexample
+namespace foo @{ namespace bar @{
+  class position;
+  class location;
+@} @}
+@end smallexample
+
+@item Accepted Values: Any absolute or relative C++ namespace reference without
+a trailing @code{"::"}.
+For example, @code{"foo"} or @code{"::foo::bar"}.
+
+@item Default Value: The value specified by @code{%name-prefix}, which defaults
+to @code{yy}.
+This usage of @code{%name-prefix} is for backward compatibility and can be
+confusing since @code{%name-prefix} also specifies the textual prefix for the
+lexical analyzer function.
+Thus, if you specify @code{%name-prefix}, it is best to also specify
+@code{%define namespace} so that @code{%name-prefix} @emph{only} affects the
+lexical analyzer function.
+For example, if you specify:
+
+@smallexample
+%define namespace "foo"
+%name-prefix "bar::"
+@end smallexample
+
+The parser namespace is @code{foo} and @code{yylex} is referenced as
+@code{bar::lex}.
+@end itemize
+@end itemize
+
+@end deffn
+
+@deffn {Directive} %defines
+Write a header file containing macro definitions for the token type
+names defined in the grammar as well as a few other declarations.
+If the parser output file is named @file{@var{name}.c} then this file
+is named @file{@var{name}.h}.
+
+For C parsers, the output header declares @code{YYSTYPE} unless
+@code{YYSTYPE} is already defined as a macro or you have used a
+@code{<@var{type}>} tag without using @code{%union}.
+Therefore, if you are using a @code{%union}
+(@pxref{Multiple Types, ,More Than One Value Type}) with components that
+require other definitions, or if you have defined a @code{YYSTYPE} macro
+or type definition
+(@pxref{Value Type, ,Data Types of Semantic Values}), you need to
+arrange for these definitions to be propagated to all modules, e.g., by
+putting them in a prerequisite header that is included both by your
+parser and by any other module that needs @code{YYSTYPE}.
+
+Unless your parser is pure, the output header declares @code{yylval}
+as an external variable.  @xref{Pure Decl, ,A Pure (Reentrant)
+Parser}.
+
+If you have also used locations, the output header declares
+@code{YYLTYPE} and @code{yylloc} using a protocol similar to that of
+the @code{YYSTYPE} macro and @code{yylval}.  @xref{Locations, ,Tracking
+Locations}.
+
+This output file is normally essential if you wish to put the definition
+of @code{yylex} in a separate source file, because @code{yylex}
+typically needs to be able to refer to the above-mentioned declarations
+and to the token type codes.  @xref{Token Values, ,Semantic Values of
+Tokens}.
+
+@findex %code requires
+@findex %code provides
+If you have declared @code{%code requires} or @code{%code provides}, the output
+header also contains their code.
+@xref{Decl Summary, ,%code}.
+@end deffn
+
+@deffn {Directive} %defines @var{defines-file}
+Same as above, but save in the file @var{defines-file}.
+@end deffn
+
+@deffn {Directive} %destructor
+Specify how the parser should reclaim the memory associated to
+discarded symbols.  @xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
+
+@deffn {Directive} %file-prefix "@var{prefix}"
+Specify a prefix to use for all Bison output file names.  The names are
+chosen as if the input file were named @file{@var{prefix}.y}.
+@end deffn
+
+@deffn {Directive} %language "@var{language}"
+Specify the programming language for the generated parser.  Currently
+supported languages include C, C++, and Java.
+@var{language} is case-insensitive.
+
+This directive is experimental and its effect may be modified in future
+releases.
+@end deffn
+
+@deffn {Directive} %locations
+Generate the code processing the locations (@pxref{Action Features,
+,Special Features for Use in Actions}).  This mode is enabled as soon as
+the grammar uses the special @samp{@@@var{n}} tokens, but if your
+grammar does not use it, using @samp{%locations} allows for more
+accurate syntax error messages.
+@end deffn
+
+@deffn {Directive} %name-prefix "@var{prefix}"
+Rename the external symbols used in the parser so that they start with
+@var{prefix} instead of @samp{yy}.  The precise list of symbols renamed
+in C parsers
+is @code{yyparse}, @code{yylex}, @code{yyerror}, @code{yynerrs},
+@code{yylval}, @code{yychar}, @code{yydebug}, and
+(if locations are used) @code{yylloc}.  If you use a push parser,
+@code{yypush_parse}, @code{yypull_parse}, @code{yypstate},
+@code{yypstate_new} and @code{yypstate_delete} will
+also be renamed.  For example, if you use @samp{%name-prefix "c_"}, the
+names become @code{c_parse}, @code{c_lex}, and so on.
+For C++ parsers, see the @code{%define namespace} documentation in this
+section.
+@xref{Multiple Parsers, ,Multiple Parsers in the Same Program}.
+@end deffn
+
+@ifset defaultprec
+@deffn {Directive} %no-default-prec
+Do not assign a precedence to rules lacking an explicit @code{%prec}
+modifier (@pxref{Contextual Precedence, ,Context-Dependent
+Precedence}).
+@end deffn
+@end ifset
+
+@deffn {Directive} %no-lines
+Don't generate any @code{#line} preprocessor commands in the parser
+file.  Ordinarily Bison writes these commands in the parser file so that
+the C compiler and debuggers will associate errors and object code with
+your source file (the grammar file).  This directive causes them to
+associate errors with the parser file, treating it an independent source
+file in its own right.
+@end deffn
+
+@deffn {Directive} %output "@var{file}"
+Specify @var{file} for the parser file.
+@end deffn
+
+@deffn {Directive} %pure-parser
+Deprecated version of @code{%define api.pure} (@pxref{Decl Summary, ,%define}),
+for which Bison is more careful to warn about unreasonable usage.
+@end deffn
+
+@deffn {Directive} %require "@var{version}"
+Require version @var{version} or higher of Bison.  @xref{Require Decl, ,
+Require a Version of Bison}.
+@end deffn
+
+@deffn {Directive} %skeleton "@var{file}"
+Specify the skeleton to use.
+
+@c You probably don't need this option unless you are developing Bison.
+@c You should use @code{%language} if you want to specify the skeleton for a
+@c different language, because it is clearer and because it will always choose the
+@c correct skeleton for non-deterministic or push parsers.
+
+If @var{file} does not contain a @code{/}, @var{file} is the name of a skeleton
+file in the Bison installation directory.
+If it does, @var{file} is an absolute file name or a file name relative to the
+directory of the grammar file.
+This is similar to how most shells resolve commands.
+@end deffn
+
+@deffn {Directive} %token-table
+Generate an array of token names in the parser file.  The name of the
+array is @code{yytname}; @code{yytname[@var{i}]} is the name of the
+token whose internal Bison token code number is @var{i}.  The first
+three elements of @code{yytname} correspond to the predefined tokens
+@code{"$end"},
+@code{"error"}, and @code{"$undefined"}; after these come the symbols
+defined in the grammar file.
+
+The name in the table includes all the characters needed to represent
+the token in Bison.  For single-character literals and literal
+strings, this includes the surrounding quoting characters and any
+escape sequences.  For example, the Bison single-character literal
+@code{'+'} corresponds to a three-character name, represented in C as
+@code{"'+'"}; and the Bison two-character literal string @code{"\\/"}
+corresponds to a five-character name, represented in C as
+@code{"\"\\\\/\""}.
+
+When you specify @code{%token-table}, Bison also generates macro
+definitions for macros @code{YYNTOKENS}, @code{YYNNTS}, and
+@code{YYNRULES}, and @code{YYNSTATES}:
+
+@table @code
+@item YYNTOKENS
+The highest token number, plus one.
+@item YYNNTS
+The number of nonterminal symbols.
+@item YYNRULES
+The number of grammar rules,
+@item YYNSTATES
+The number of parser states (@pxref{Parser States}).
+@end table
+@end deffn
+
+@deffn {Directive} %verbose
+Write an extra output file containing verbose descriptions of the
+parser states and what is done for each type of lookahead token in
+that state.  @xref{Understanding, , Understanding Your Parser}, for more
+information.
+@end deffn
+
+@deffn {Directive} %yacc
+Pretend the option @option{--yacc} was given, i.e., imitate Yacc,
+including its naming conventions.  @xref{Bison Options}, for more.
+@end deffn
+
+
+@node Multiple Parsers
+@section Multiple Parsers in the Same Program
+
+Most programs that use Bison parse only one language and therefore contain
+only one Bison parser.  But what if you want to parse more than one
+language with the same program?  Then you need to avoid a name conflict
+between different definitions of @code{yyparse}, @code{yylval}, and so on.
+
+The easy way to do this is to use the option @samp{-p @var{prefix}}
+(@pxref{Invocation, ,Invoking Bison}).  This renames the interface
+functions and variables of the Bison parser to start with @var{prefix}
+instead of @samp{yy}.  You can use this to give each parser distinct
+names that do not conflict.
+
+The precise list of symbols renamed is @code{yyparse}, @code{yylex},
+@code{yyerror}, @code{yynerrs}, @code{yylval}, @code{yylloc},
+@code{yychar} and @code{yydebug}.  If you use a push parser,
+@code{yypush_parse}, @code{yypull_parse}, @code{yypstate},
+@code{yypstate_new} and @code{yypstate_delete} will also be renamed.
+For example, if you use @samp{-p c}, the names become @code{cparse},
+@code{clex}, and so on.
+
+@strong{All the other variables and macros associated with Bison are not
+renamed.} These others are not global; there is no conflict if the same
+name is used in different parsers.  For example, @code{YYSTYPE} is not
+renamed, but defining this in different ways in different parsers causes
+no trouble (@pxref{Value Type, ,Data Types of Semantic Values}).
+
+The @samp{-p} option works by adding macro definitions to the beginning
+of the parser source file, defining @code{yyparse} as
+@code{@var{prefix}parse}, and so on.  This effectively substitutes one
+name for the other in the entire parser file.
+
+@node Interface
+@chapter Parser C-Language Interface
+@cindex C-language interface
+@cindex interface
+
+The Bison parser is actually a C function named @code{yyparse}.  Here we
+describe the interface conventions of @code{yyparse} and the other
+functions that it needs to use.
+
+Keep in mind that the parser uses many C identifiers starting with
+@samp{yy} and @samp{YY} for internal purposes.  If you use such an
+identifier (aside from those in this manual) in an action or in epilogue
+in the grammar file, you are likely to run into trouble.
+
+@menu
+* Parser Function::         How to call @code{yyparse} and what it returns.
+* Push Parser Function::    How to call @code{yypush_parse} and what it returns.
+* Pull Parser Function::    How to call @code{yypull_parse} and what it returns.
+* Parser Create Function::  How to call @code{yypstate_new} and what it returns.
+* Parser Delete Function::  How to call @code{yypstate_delete} and what it returns.
+* Lexical::                 You must supply a function @code{yylex}
+                              which reads tokens.
+* Error Reporting::         You must supply a function @code{yyerror}.
+* Action Features::         Special features for use in actions.
+* Internationalization::    How to let the parser speak in the user's
+                              native language.
+@end menu
+
+@node Parser Function
+@section The Parser Function @code{yyparse}
+@findex yyparse
+
+You call the function @code{yyparse} to cause parsing to occur.  This
+function reads tokens, executes actions, and ultimately returns when it
+encounters end-of-input or an unrecoverable syntax error.  You can also
+write an action which directs @code{yyparse} to return immediately
+without reading further.
+
+
+@deftypefun int yyparse (void)
+The value returned by @code{yyparse} is 0 if parsing was successful (return
+is due to end-of-input).
+
+The value is 1 if parsing failed because of invalid input, i.e., input
+that contains a syntax error or that causes @code{YYABORT} to be
+invoked.
+
+The value is 2 if parsing failed due to memory exhaustion.
+@end deftypefun
+
+In an action, you can cause immediate return from @code{yyparse} by using
+these macros:
+
+@defmac YYACCEPT
+@findex YYACCEPT
+Return immediately with value 0 (to report success).
+@end defmac
+
+@defmac YYABORT
+@findex YYABORT
+Return immediately with value 1 (to report failure).
+@end defmac
+
+If you use a reentrant parser, you can optionally pass additional
+parameter information to it in a reentrant way.  To do so, use the
+declaration @code{%parse-param}:
+
+@deffn {Directive} %parse-param @{@var{argument-declaration}@}
+@findex %parse-param
+Declare that an argument declared by the braced-code
+@var{argument-declaration} is an additional @code{yyparse} argument.
+The @var{argument-declaration} is used when declaring
+functions or prototypes.  The last identifier in
+@var{argument-declaration} must be the argument name.
+@end deffn
+
+Here's an example.  Write this in the parser:
+
+@example
+%parse-param @{int *nastiness@}
+%parse-param @{int *randomness@}
+@end example
+
+@noindent
+Then call the parser like this:
+
+@example
+@{
+  int nastiness, randomness;
+  @dots{}  /* @r{Store proper data in @code{nastiness} and @code{randomness}.}  */
+  value = yyparse (&nastiness, &randomness);
+  @dots{}
+@}
+@end example
+
+@noindent
+In the grammar actions, use expressions like this to refer to the data:
+
+@example
+exp: @dots{}    @{ @dots{}; *randomness += 1; @dots{} @}
+@end example
+
+@node Push Parser Function
+@section The Push Parser Function @code{yypush_parse}
+@findex yypush_parse
+
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+
+You call the function @code{yypush_parse} to parse a single token.  This
+function is available if either the @code{%define api.push_pull "push"} or
+@code{%define api.push_pull "both"} declaration is used.
+@xref{Push Decl, ,A Push Parser}.
+
+@deftypefun int yypush_parse (yypstate *yyps)
+The value returned by @code{yypush_parse} is the same as for yyparse with the
+following exception.  @code{yypush_parse} will return YYPUSH_MORE if more input
+is required to finish parsing the grammar.
+@end deftypefun
+
+@node Pull Parser Function
+@section The Pull Parser Function @code{yypull_parse}
+@findex yypull_parse
+
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+
+You call the function @code{yypull_parse} to parse the rest of the input
+stream.  This function is available if the @code{%define api.push_pull "both"}
+declaration is used.
+@xref{Push Decl, ,A Push Parser}.
+
+@deftypefun int yypull_parse (yypstate *yyps)
+The value returned by @code{yypull_parse} is the same as for @code{yyparse}.
+@end deftypefun
+
+@node Parser Create Function
+@section The Parser Create Function @code{yystate_new}
+@findex yypstate_new
+
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+
+You call the function @code{yypstate_new} to create a new parser instance.
+This function is available if either the @code{%define api.push_pull "push"} or
+@code{%define api.push_pull "both"} declaration is used.
+@xref{Push Decl, ,A Push Parser}.
+
+@deftypefun yypstate *yypstate_new (void)
+The fuction will return a valid parser instance if there was memory available
+or 0 if no memory was available.
+In impure mode, it will also return 0 if a parser instance is currently
+allocated.
+@end deftypefun
+
+@node Parser Delete Function
+@section The Parser Delete Function @code{yystate_delete}
+@findex yypstate_delete
+
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+
+You call the function @code{yypstate_delete} to delete a parser instance.
+function is available if either the @code{%define api.push_pull "push"} or
+@code{%define api.push_pull "both"} declaration is used.
+@xref{Push Decl, ,A Push Parser}.
+
+@deftypefun void yypstate_delete (yypstate *yyps)
+This function will reclaim the memory associated with a parser instance.
+After this call, you should no longer attempt to use the parser instance.
+@end deftypefun
+
+@node Lexical
+@section The Lexical Analyzer Function @code{yylex}
+@findex yylex
+@cindex lexical analyzer
+
+The @dfn{lexical analyzer} function, @code{yylex}, recognizes tokens from
+the input stream and returns them to the parser.  Bison does not create
+this function automatically; you must write it so that @code{yyparse} can
+call it.  The function is sometimes referred to as a lexical scanner.
+
+In simple programs, @code{yylex} is often defined at the end of the Bison
+grammar file.  If @code{yylex} is defined in a separate source file, you
+need to arrange for the token-type macro definitions to be available there.
+To do this, use the @samp{-d} option when you run Bison, so that it will
+write these macro definitions into a separate header file
+@file{@var{name}.tab.h} which you can include in the other source files
+that need it.  @xref{Invocation, ,Invoking Bison}.
+
+@menu
+* Calling Convention::  How @code{yyparse} calls @code{yylex}.
+* Token Values::        How @code{yylex} must return the semantic value
+                          of the token it has read.
+* Token Locations::     How @code{yylex} must return the text location
+                          (line number, etc.) of the token, if the
+                          actions want that.
+* Pure Calling::        How the calling convention differs in a pure parser
+                          (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}).
+@end menu
+
+@node Calling Convention
+@subsection Calling Convention for @code{yylex}
+
+The value that @code{yylex} returns must be the positive numeric code
+for the type of token it has just found; a zero or negative value
+signifies end-of-input.
+
+When a token is referred to in the grammar rules by a name, that name
+in the parser file becomes a C macro whose definition is the proper
+numeric code for that token type.  So @code{yylex} can use the name
+to indicate that type.  @xref{Symbols}.
+
+When a token is referred to in the grammar rules by a character literal,
+the numeric code for that character is also the code for the token type.
+So @code{yylex} can simply return that character code, possibly converted
+to @code{unsigned char} to avoid sign-extension.  The null character
+must not be used this way, because its code is zero and that
+signifies end-of-input.
+
+Here is an example showing these things:
+
+@example
+int
+yylex (void)
+@{
+  @dots{}
+  if (c == EOF)    /* Detect end-of-input.  */
+    return 0;
+  @dots{}
+  if (c == '+' || c == '-')
+    return c;      /* Assume token type for `+' is '+'.  */
+  @dots{}
+  return INT;      /* Return the type of the token.  */
+  @dots{}
+@}
+@end example
+
+@noindent
+This interface has been designed so that the output from the @code{lex}
+utility can be used without change as the definition of @code{yylex}.
+
+If the grammar uses literal string tokens, there are two ways that
+@code{yylex} can determine the token type codes for them:
+
+@itemize @bullet
+@item
+If the grammar defines symbolic token names as aliases for the
+literal string tokens, @code{yylex} can use these symbolic names like
+all others.  In this case, the use of the literal string tokens in
+the grammar file has no effect on @code{yylex}.
+
+@item
+@code{yylex} can find the multicharacter token in the @code{yytname}
+table.  The index of the token in the table is the token type's code.
+The name of a multicharacter token is recorded in @code{yytname} with a
+double-quote, the token's characters, and another double-quote.  The
+token's characters are escaped as necessary to be suitable as input
+to Bison.
+
+Here's code for looking up a multicharacter token in @code{yytname},
+assuming that the characters of the token are stored in
+@code{token_buffer}, and assuming that the token does not contain any
+characters like @samp{"} that require escaping.
+
+@smallexample
+for (i = 0; i < YYNTOKENS; i++)
+  @{
+    if (yytname[i] != 0
+        && yytname[i][0] == '"'
+        && ! strncmp (yytname[i] + 1, token_buffer,
+                      strlen (token_buffer))
+        && yytname[i][strlen (token_buffer) + 1] == '"'
+        && yytname[i][strlen (token_buffer) + 2] == 0)
+      break;
+  @}
+@end smallexample
+
+The @code{yytname} table is generated only if you use the
+@code{%token-table} declaration.  @xref{Decl Summary}.
+@end itemize
+
+@node Token Values
+@subsection Semantic Values of Tokens
+
+@vindex yylval
+In an ordinary (nonreentrant) parser, the semantic value of the token must
+be stored into the global variable @code{yylval}.  When you are using
+just one data type for semantic values, @code{yylval} has that type.
+Thus, if the type is @code{int} (the default), you might write this in
+@code{yylex}:
+
+@example
+@group
+  @dots{}
+  yylval = value;  /* Put value onto Bison stack.  */
+  return INT;      /* Return the type of the token.  */
+  @dots{}
+@end group
+@end example
+
+When you are using multiple data types, @code{yylval}'s type is a union
+made from the @code{%union} declaration (@pxref{Union Decl, ,The
+Collection of Value Types}).  So when you store a token's value, you
+must use the proper member of the union.  If the @code{%union}
+declaration looks like this:
+
+@example
+@group
+%union @{
+  int intval;
+  double val;
+  symrec *tptr;
+@}
+@end group
+@end example
+
+@noindent
+then the code in @code{yylex} might look like this:
+
+@example
+@group
+  @dots{}
+  yylval.intval = value; /* Put value onto Bison stack.  */
+  return INT;            /* Return the type of the token.  */
+  @dots{}
+@end group
+@end example
+
+@node Token Locations
+@subsection Textual Locations of Tokens
+
+@vindex yylloc
+If you are using the @samp{@@@var{n}}-feature (@pxref{Locations, ,
+Tracking Locations}) in actions to keep track of the textual locations
+of tokens and groupings, then you must provide this information in
+@code{yylex}.  The function @code{yyparse} expects to find the textual
+location of a token just parsed in the global variable @code{yylloc}.
+So @code{yylex} must store the proper data in that variable.
+
+By default, the value of @code{yylloc} is a structure and you need only
+initialize the members that are going to be used by the actions.  The
+four members are called @code{first_line}, @code{first_column},
+@code{last_line} and @code{last_column}.  Note that the use of this
+feature makes the parser noticeably slower.
+
+@tindex YYLTYPE
+The data type of @code{yylloc} has the name @code{YYLTYPE}.
+
+@node Pure Calling
+@subsection Calling Conventions for Pure Parsers
+
+When you use the Bison declaration @code{%define api.pure} to request a
+pure, reentrant parser, the global communication variables @code{yylval}
+and @code{yylloc} cannot be used.  (@xref{Pure Decl, ,A Pure (Reentrant)
+Parser}.)  In such parsers the two global variables are replaced by
+pointers passed as arguments to @code{yylex}.  You must declare them as
+shown here, and pass the information back by storing it through those
+pointers.
+
+@example
+int
+yylex (YYSTYPE *lvalp, YYLTYPE *llocp)
+@{
+  @dots{}
+  *lvalp = value;  /* Put value onto Bison stack.  */
+  return INT;      /* Return the type of the token.  */
+  @dots{}
+@}
+@end example
+
+If the grammar file does not use the @samp{@@} constructs to refer to
+textual locations, then the type @code{YYLTYPE} will not be defined.  In
+this case, omit the second argument; @code{yylex} will be called with
+only one argument.
+
+
+If you wish to pass the additional parameter data to @code{yylex}, use
+@code{%lex-param} just like @code{%parse-param} (@pxref{Parser
+Function}).
+
+@deffn {Directive} lex-param @{@var{argument-declaration}@}
+@findex %lex-param
+Declare that the braced-code @var{argument-declaration} is an
+additional @code{yylex} argument declaration.
+@end deffn
+
+For instance:
+
+@example
+%parse-param @{int *nastiness@}
+%lex-param   @{int *nastiness@}
+%parse-param @{int *randomness@}
+@end example
+
+@noindent
+results in the following signature:
+
+@example
+int yylex   (int *nastiness);
+int yyparse (int *nastiness, int *randomness);
+@end example
+
+If @code{%define api.pure} is added:
+
+@example
+int yylex   (YYSTYPE *lvalp, int *nastiness);
+int yyparse (int *nastiness, int *randomness);
+@end example
+
+@noindent
+and finally, if both @code{%define api.pure} and @code{%locations} are used:
+
+@example
+int yylex   (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
+int yyparse (int *nastiness, int *randomness);
+@end example
+
+@node Error Reporting
+@section The Error Reporting Function @code{yyerror}
+@cindex error reporting function
+@findex yyerror
+@cindex parse error
+@cindex syntax error
+
+The Bison parser detects a @dfn{syntax error} or @dfn{parse error}
+whenever it reads a token which cannot satisfy any syntax rule.  An
+action in the grammar can also explicitly proclaim an error, using the
+macro @code{YYERROR} (@pxref{Action Features, ,Special Features for Use
+in Actions}).
+
+The Bison parser expects to report the error by calling an error
+reporting function named @code{yyerror}, which you must supply.  It is
+called by @code{yyparse} whenever a syntax error is found, and it
+receives one argument.  For a syntax error, the string is normally
+@w{@code{"syntax error"}}.
+
+@findex %error-verbose
+If you invoke the directive @code{%error-verbose} in the Bison
+declarations section (@pxref{Bison Declarations, ,The Bison Declarations
+Section}), then Bison provides a more verbose and specific error message
+string instead of just plain @w{@code{"syntax error"}}.
+
+The parser can detect one other kind of error: memory exhaustion.  This
+can happen when the input contains constructions that are very deeply
+nested.  It isn't likely you will encounter this, since the Bison
+parser normally extends its stack automatically up to a very large limit.  But
+if memory is exhausted, @code{yyparse} calls @code{yyerror} in the usual
+fashion, except that the argument string is @w{@code{"memory exhausted"}}.
+
+In some cases diagnostics like @w{@code{"syntax error"}} are
+translated automatically from English to some other language before
+they are passed to @code{yyerror}.  @xref{Internationalization}.
+
+The following definition suffices in simple programs:
+
+@example
+@group
+void
+yyerror (char const *s)
+@{
+@end group
+@group
+  fprintf (stderr, "%s\n", s);
+@}
+@end group
+@end example
+
+After @code{yyerror} returns to @code{yyparse}, the latter will attempt
+error recovery if you have written suitable error recovery grammar rules
+(@pxref{Error Recovery}).  If recovery is impossible, @code{yyparse} will
+immediately return 1.
+
+Obviously, in location tracking pure parsers, @code{yyerror} should have
+an access to the current location.
+This is indeed the case for the @acronym{GLR}
+parsers, but not for the Yacc parser, for historical reasons.  I.e., if
+@samp{%locations %define api.pure} is passed then the prototypes for
+@code{yyerror} are:
+
+@example
+void yyerror (char const *msg);                 /* Yacc parsers.  */
+void yyerror (YYLTYPE *locp, char const *msg);  /* GLR parsers.   */
+@end example
+
+If @samp{%parse-param @{int *nastiness@}} is used, then:
+
+@example
+void yyerror (int *nastiness, char const *msg);  /* Yacc parsers.  */
+void yyerror (int *nastiness, char const *msg);  /* GLR parsers.   */
+@end example
+
+Finally, @acronym{GLR} and Yacc parsers share the same @code{yyerror} calling
+convention for absolutely pure parsers, i.e., when the calling
+convention of @code{yylex} @emph{and} the calling convention of
+@code{%define api.pure} are pure.
+I.e.:
+
+@example
+/* Location tracking.  */
+%locations
+/* Pure yylex.  */
+%define api.pure
+%lex-param   @{int *nastiness@}
+/* Pure yyparse.  */
+%parse-param @{int *nastiness@}
+%parse-param @{int *randomness@}
+@end example
+
+@noindent
+results in the following signatures for all the parser kinds:
+
+@example
+int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
+int yyparse (int *nastiness, int *randomness);
+void yyerror (YYLTYPE *locp,
+              int *nastiness, int *randomness,
+              char const *msg);
+@end example
+
+@noindent
+The prototypes are only indications of how the code produced by Bison
+uses @code{yyerror}.  Bison-generated code always ignores the returned
+value, so @code{yyerror} can return any type, including @code{void}.
+Also, @code{yyerror} can be a variadic function; that is why the
+message is always passed last.
+
+Traditionally @code{yyerror} returns an @code{int} that is always
+ignored, but this is purely for historical reasons, and @code{void} is
+preferable since it more accurately describes the return type for
+@code{yyerror}.
+
+@vindex yynerrs
+The variable @code{yynerrs} contains the number of syntax errors
+reported so far.  Normally this variable is global; but if you
+request a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser})
+then it is a local variable which only the actions can access.
+
+@node Action Features
+@section Special Features for Use in Actions
+@cindex summary, action features
+@cindex action features summary
+
+Here is a table of Bison constructs, variables and macros that
+are useful in actions.
+
+@deffn {Variable} $$
+Acts like a variable that contains the semantic value for the
+grouping made by the current rule.  @xref{Actions}.
+@end deffn
+
+@deffn {Variable} $@var{n}
+Acts like a variable that contains the semantic value for the
+@var{n}th component of the current rule.  @xref{Actions}.
+@end deffn
+
+@deffn {Variable} $<@var{typealt}>$
+Like @code{$$} but specifies alternative @var{typealt} in the union
+specified by the @code{%union} declaration.  @xref{Action Types, ,Data
+Types of Values in Actions}.
+@end deffn
+
+@deffn {Variable} $<@var{typealt}>@var{n}
+Like @code{$@var{n}} but specifies alternative @var{typealt} in the
+union specified by the @code{%union} declaration.
+@xref{Action Types, ,Data Types of Values in Actions}.
+@end deffn
+
+@deffn {Macro} YYABORT;
+Return immediately from @code{yyparse}, indicating failure.
+@xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@end deffn
+
+@deffn {Macro} YYACCEPT;
+Return immediately from @code{yyparse}, indicating success.
+@xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@end deffn
+
+@deffn {Macro} YYBACKUP (@var{token}, @var{value});
+@findex YYBACKUP
+Unshift a token.  This macro is allowed only for rules that reduce
+a single value, and only when there is no lookahead token.
+It is also disallowed in @acronym{GLR} parsers.
+It installs a lookahead token with token type @var{token} and
+semantic value @var{value}; then it discards the value that was
+going to be reduced by this rule.
+
+If the macro is used when it is not valid, such as when there is
+a lookahead token already, then it reports a syntax error with
+a message @samp{cannot back up} and performs ordinary error
+recovery.
+
+In either case, the rest of the action is not executed.
+@end deffn
+
+@deffn {Macro} YYEMPTY
+@vindex YYEMPTY
+Value stored in @code{yychar} when there is no lookahead token.
+@end deffn
+
+@deffn {Macro} YYEOF
+@vindex YYEOF
+Value stored in @code{yychar} when the lookahead is the end of the input
+stream.
+@end deffn
+
+@deffn {Macro} YYERROR;
+@findex YYERROR
+Cause an immediate syntax error.  This statement initiates error
+recovery just as if the parser itself had detected an error; however, it
+does not call @code{yyerror}, and does not print any message.  If you
+want to print an error message, call @code{yyerror} explicitly before
+the @samp{YYERROR;} statement.  @xref{Error Recovery}.
+@end deffn
+
+@deffn {Macro} YYRECOVERING
+@findex YYRECOVERING
+The expression @code{YYRECOVERING ()} yields 1 when the parser
+is recovering from a syntax error, and 0 otherwise.
+@xref{Error Recovery}.
+@end deffn
+
+@deffn {Variable} yychar
+Variable containing either the lookahead token, or @code{YYEOF} when the
+lookahead is the end of the input stream, or @code{YYEMPTY} when no lookahead
+has been performed so the next token is not yet known.
+Do not modify @code{yychar} in a deferred semantic action (@pxref{GLR Semantic
+Actions}).
+@xref{Lookahead, ,Lookahead Tokens}.
+@end deffn
+
+@deffn {Macro} yyclearin;
+Discard the current lookahead token.  This is useful primarily in
+error rules.
+Do not invoke @code{yyclearin} in a deferred semantic action (@pxref{GLR
+Semantic Actions}).
+@xref{Error Recovery}.
+@end deffn
+
+@deffn {Macro} yyerrok;
+Resume generating error messages immediately for subsequent syntax
+errors.  This is useful primarily in error rules.
+@xref{Error Recovery}.
+@end deffn
+
+@deffn {Variable} yylloc
+Variable containing the lookahead token location when @code{yychar} is not set
+to @code{YYEMPTY} or @code{YYEOF}.
+Do not modify @code{yylloc} in a deferred semantic action (@pxref{GLR Semantic
+Actions}).
+@xref{Actions and Locations, ,Actions and Locations}.
+@end deffn
+
+@deffn {Variable} yylval
+Variable containing the lookahead token semantic value when @code{yychar} is
+not set to @code{YYEMPTY} or @code{YYEOF}.
+Do not modify @code{yylval} in a deferred semantic action (@pxref{GLR Semantic
+Actions}).
+@xref{Actions, ,Actions}.
+@end deffn
+
+@deffn {Value} @@$
+@findex @@$
+Acts like a structure variable containing information on the textual location
+of the grouping made by the current rule.  @xref{Locations, ,
+Tracking Locations}.
+
+@c Check if those paragraphs are still useful or not.
+
+@c @example
+@c struct @{
+@c   int first_line, last_line;
+@c   int first_column, last_column;
+@c @};
+@c @end example
+
+@c Thus, to get the starting line number of the third component, you would
+@c use @samp{@@3.first_line}.
+
+@c In order for the members of this structure to contain valid information,
+@c you must make @code{yylex} supply this information about each token.
+@c If you need only certain members, then @code{yylex} need only fill in
+@c those members.
+
+@c The use of this feature makes the parser noticeably slower.
+@end deffn
+
+@deffn {Value} @@@var{n}
+@findex @@@var{n}
+Acts like a structure variable containing information on the textual location
+of the @var{n}th component of the current rule.  @xref{Locations, ,
+Tracking Locations}.
+@end deffn
+
+@node Internationalization
+@section Parser Internationalization
+@cindex internationalization
+@cindex i18n
+@cindex NLS
+@cindex gettext
+@cindex bison-po
+
+A Bison-generated parser can print diagnostics, including error and
+tracing messages.  By default, they appear in English.  However, Bison
+also supports outputting diagnostics in the user's native language.  To
+make this work, the user should set the usual environment variables.
+@xref{Users, , The User's View, gettext, GNU @code{gettext} utilities}.
+For example, the shell command @samp{export LC_ALL=fr_CA.UTF-8} might
+set the user's locale to French Canadian using the @acronym{UTF}-8
+encoding.  The exact set of available locales depends on the user's
+installation.
+
+The maintainer of a package that uses a Bison-generated parser enables
+the internationalization of the parser's output through the following
+steps.  Here we assume a package that uses @acronym{GNU} Autoconf and
+@acronym{GNU} Automake.
+
+@enumerate
+@item
+@cindex bison-i18n.m4
+Into the directory containing the @acronym{GNU} Autoconf macros used
+by the package---often called @file{m4}---copy the
+@file{bison-i18n.m4} file installed by Bison under
+@samp{share/aclocal/bison-i18n.m4} in Bison's installation directory.
+For example:
+
+@example
+cp /usr/local/share/aclocal/bison-i18n.m4 m4/bison-i18n.m4
+@end example
+
+@item
+@findex BISON_I18N
+@vindex BISON_LOCALEDIR
+@vindex YYENABLE_NLS
+In the top-level @file{configure.ac}, after the @code{AM_GNU_GETTEXT}
+invocation, add an invocation of @code{BISON_I18N}.  This macro is
+defined in the file @file{bison-i18n.m4} that you copied earlier.  It
+causes @samp{configure} to find the value of the
+@code{BISON_LOCALEDIR} variable, and it defines the source-language
+symbol @code{YYENABLE_NLS} to enable translations in the
+Bison-generated parser.
+
+@item
+In the @code{main} function of your program, designate the directory
+containing Bison's runtime message catalog, through a call to
+@samp{bindtextdomain} with domain name @samp{bison-runtime}.
+For example:
+
+@example
+bindtextdomain ("bison-runtime", BISON_LOCALEDIR);
+@end example
+
+Typically this appears after any other call @code{bindtextdomain
+(PACKAGE, LOCALEDIR)} that your package already has.  Here we rely on
+@samp{BISON_LOCALEDIR} to be defined as a string through the
+@file{Makefile}.
+
+@item
+In the @file{Makefile.am} that controls the compilation of the @code{main}
+function, make @samp{BISON_LOCALEDIR} available as a C preprocessor macro,
+either in @samp{DEFS} or in @samp{AM_CPPFLAGS}.  For example:
+
+@example
+DEFS = @@DEFS@@ -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
+@end example
+
+or:
+
+@example
+AM_CPPFLAGS = -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
+@end example
+
+@item
+Finally, invoke the command @command{autoreconf} to generate the build
+infrastructure.
+@end enumerate
+
+
+@node Algorithm
+@chapter The Bison Parser Algorithm
+@cindex Bison parser algorithm
+@cindex algorithm of parser
+@cindex shifting
+@cindex reduction
+@cindex parser stack
+@cindex stack, parser
+
+As Bison reads tokens, it pushes them onto a stack along with their
+semantic values.  The stack is called the @dfn{parser stack}.  Pushing a
+token is traditionally called @dfn{shifting}.
+
+For example, suppose the infix calculator has read @samp{1 + 5 *}, with a
+@samp{3} to come.  The stack will have four elements, one for each token
+that was shifted.
+
+But the stack does not always have an element for each token read.  When
+the last @var{n} tokens and groupings shifted match the components of a
+grammar rule, they can be combined according to that rule.  This is called
+@dfn{reduction}.  Those tokens and groupings are replaced on the stack by a
+single grouping whose symbol is the result (left hand side) of that rule.
+Running the rule's action is part of the process of reduction, because this
+is what computes the semantic value of the resulting grouping.
+
+For example, if the infix calculator's parser stack contains this:
+
+@example
+1 + 5 * 3
+@end example
+
+@noindent
+and the next input token is a newline character, then the last three
+elements can be reduced to 15 via the rule:
+
+@example
+expr: expr '*' expr;
+@end example
+
+@noindent
+Then the stack contains just these three elements:
+
+@example
+1 + 15
+@end example
+
+@noindent
+At this point, another reduction can be made, resulting in the single value
+16.  Then the newline token can be shifted.
+
+The parser tries, by shifts and reductions, to reduce the entire input down
+to a single grouping whose symbol is the grammar's start-symbol
+(@pxref{Language and Grammar, ,Languages and Context-Free Grammars}).
+
+This kind of parser is known in the literature as a bottom-up parser.
+
+@menu
+* Lookahead::         Parser looks one token ahead when deciding what to do.
+* Shift/Reduce::      Conflicts: when either shifting or reduction is valid.
+* Precedence::        Operator precedence works by resolving conflicts.
+* Contextual Precedence::  When an operator's precedence depends on context.
+* Parser States::     The parser is a finite-state-machine with stack.
+* Reduce/Reduce::     When two rules are applicable in the same situation.
+* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified.
+* Generalized LR Parsing::  Parsing arbitrary context-free grammars.
+* Memory Management:: What happens when memory is exhausted.  How to avoid it.
+@end menu
+
+@node Lookahead
+@section Lookahead Tokens
+@cindex lookahead token
+
+The Bison parser does @emph{not} always reduce immediately as soon as the
+last @var{n} tokens and groupings match a rule.  This is because such a
+simple strategy is inadequate to handle most languages.  Instead, when a
+reduction is possible, the parser sometimes ``looks ahead'' at the next
+token in order to decide what to do.
+
+When a token is read, it is not immediately shifted; first it becomes the
+@dfn{lookahead token}, which is not on the stack.  Now the parser can
+perform one or more reductions of tokens and groupings on the stack, while
+the lookahead token remains off to the side.  When no more reductions
+should take place, the lookahead token is shifted onto the stack.  This
+does not mean that all possible reductions have been done; depending on the
+token type of the lookahead token, some rules may choose to delay their
+application.
+
+Here is a simple case where lookahead is needed.  These three rules define
+expressions which contain binary addition operators and postfix unary
+factorial operators (@samp{!}), and allow parentheses for grouping.
+
+@example
+@group
+expr:     term '+' expr
+        | term
+        ;
+@end group
+
+@group
+term:     '(' expr ')'
+        | term '!'
+        | NUMBER
+        ;
+@end group
+@end example
+
+Suppose that the tokens @w{@samp{1 + 2}} have been read and shifted; what
+should be done?  If the following token is @samp{)}, then the first three
+tokens must be reduced to form an @code{expr}.  This is the only valid
+course, because shifting the @samp{)} would produce a sequence of symbols
+@w{@code{term ')'}}, and no rule allows this.
+
+If the following token is @samp{!}, then it must be shifted immediately so
+that @w{@samp{2 !}} can be reduced to make a @code{term}.  If instead the
+parser were to reduce before shifting, @w{@samp{1 + 2}} would become an
+@code{expr}.  It would then be impossible to shift the @samp{!} because
+doing so would produce on the stack the sequence of symbols @code{expr
+'!'}.  No rule allows that sequence.
+
+@vindex yychar
+@vindex yylval
+@vindex yylloc
+The lookahead token is stored in the variable @code{yychar}.
+Its semantic value and location, if any, are stored in the variables
+@code{yylval} and @code{yylloc}.
+@xref{Action Features, ,Special Features for Use in Actions}.
+
+@node Shift/Reduce
+@section Shift/Reduce Conflicts
+@cindex conflicts
+@cindex shift/reduce conflicts
+@cindex dangling @code{else}
+@cindex @code{else}, dangling
+
+Suppose we are parsing a language which has if-then and if-then-else
+statements, with a pair of rules like this:
+
+@example
+@group
+if_stmt:
+          IF expr THEN stmt
+        | IF expr THEN stmt ELSE stmt
+        ;
+@end group
+@end example
+
+@noindent
+Here we assume that @code{IF}, @code{THEN} and @code{ELSE} are
+terminal symbols for specific keyword tokens.
+
+When the @code{ELSE} token is read and becomes the lookahead token, the
+contents of the stack (assuming the input is valid) are just right for
+reduction by the first rule.  But it is also legitimate to shift the
+@code{ELSE}, because that would lead to eventual reduction by the second
+rule.
+
+This situation, where either a shift or a reduction would be valid, is
+called a @dfn{shift/reduce conflict}.  Bison is designed to resolve
+these conflicts by choosing to shift, unless otherwise directed by
+operator precedence declarations.  To see the reason for this, let's
+contrast it with the other alternative.
+
+Since the parser prefers to shift the @code{ELSE}, the result is to attach
+the else-clause to the innermost if-statement, making these two inputs
+equivalent:
+
+@example
+if x then if y then win (); else lose;
+
+if x then do; if y then win (); else lose; end;
+@end example
+
+But if the parser chose to reduce when possible rather than shift, the
+result would be to attach the else-clause to the outermost if-statement,
+making these two inputs equivalent:
+
+@example
+if x then if y then win (); else lose;
+
+if x then do; if y then win (); end; else lose;
+@end example
+
+The conflict exists because the grammar as written is ambiguous: either
+parsing of the simple nested if-statement is legitimate.  The established
+convention is that these ambiguities are resolved by attaching the
+else-clause to the innermost if-statement; this is what Bison accomplishes
+by choosing to shift rather than reduce.  (It would ideally be cleaner to
+write an unambiguous grammar, but that is very hard to do in this case.)
+This particular ambiguity was first encountered in the specifications of
+Algol 60 and is called the ``dangling @code{else}'' ambiguity.
+
+To avoid warnings from Bison about predictable, legitimate shift/reduce
+conflicts, use the @code{%expect @var{n}} declaration.  There will be no
+warning as long as the number of shift/reduce conflicts is exactly @var{n}.
+@xref{Expect Decl, ,Suppressing Conflict Warnings}.
+
+The definition of @code{if_stmt} above is solely to blame for the
+conflict, but the conflict does not actually appear without additional
+rules.  Here is a complete Bison input file that actually manifests the
+conflict:
+
+@example
+@group
+%token IF THEN ELSE variable
+%%
+@end group
+@group
+stmt:     expr
+        | if_stmt
+        ;
+@end group
+
+@group
+if_stmt:
+          IF expr THEN stmt
+        | IF expr THEN stmt ELSE stmt
+        ;
+@end group
+
+expr:     variable
+        ;
+@end example
+
+@node Precedence
+@section Operator Precedence
+@cindex operator precedence
+@cindex precedence of operators
+
+Another situation where shift/reduce conflicts appear is in arithmetic
+expressions.  Here shifting is not always the preferred resolution; the
+Bison declarations for operator precedence allow you to specify when to
+shift and when to reduce.
+
+@menu
+* Why Precedence::    An example showing why precedence is needed.
+* Using Precedence::  How to specify precedence in Bison grammars.
+* Precedence Examples::  How these features are used in the previous example.
+* How Precedence::    How they work.
+@end menu
+
+@node Why Precedence
+@subsection When Precedence is Needed
+
+Consider the following ambiguous grammar fragment (ambiguous because the
+input @w{@samp{1 - 2 * 3}} can be parsed in two different ways):
+
+@example
+@group
+expr:     expr '-' expr
+        | expr '*' expr
+        | expr '<' expr
+        | '(' expr ')'
+        @dots{}
+        ;
+@end group
+@end example
+
+@noindent
+Suppose the parser has seen the tokens @samp{1}, @samp{-} and @samp{2};
+should it reduce them via the rule for the subtraction operator?  It
+depends on the next token.  Of course, if the next token is @samp{)}, we
+must reduce; shifting is invalid because no single rule can reduce the
+token sequence @w{@samp{- 2 )}} or anything starting with that.  But if
+the next token is @samp{*} or @samp{<}, we have a choice: either
+shifting or reduction would allow the parse to complete, but with
+different results.
+
+To decide which one Bison should do, we must consider the results.  If
+the next operator token @var{op} is shifted, then it must be reduced
+first in order to permit another opportunity to reduce the difference.
+The result is (in effect) @w{@samp{1 - (2 @var{op} 3)}}.  On the other
+hand, if the subtraction is reduced before shifting @var{op}, the result
+is @w{@samp{(1 - 2) @var{op} 3}}.  Clearly, then, the choice of shift or
+reduce should depend on the relative precedence of the operators
+@samp{-} and @var{op}: @samp{*} should be shifted first, but not
+@samp{<}.
+
+@cindex associativity
+What about input such as @w{@samp{1 - 2 - 5}}; should this be
+@w{@samp{(1 - 2) - 5}} or should it be @w{@samp{1 - (2 - 5)}}?  For most
+operators we prefer the former, which is called @dfn{left association}.
+The latter alternative, @dfn{right association}, is desirable for
+assignment operators.  The choice of left or right association is a
+matter of whether the parser chooses to shift or reduce when the stack
+contains @w{@samp{1 - 2}} and the lookahead token is @samp{-}: shifting
+makes right-associativity.
+
+@node Using Precedence
+@subsection Specifying Operator Precedence
+@findex %left
+@findex %right
+@findex %nonassoc
+
+Bison allows you to specify these choices with the operator precedence
+declarations @code{%left} and @code{%right}.  Each such declaration
+contains a list of tokens, which are operators whose precedence and
+associativity is being declared.  The @code{%left} declaration makes all
+those operators left-associative and the @code{%right} declaration makes
+them right-associative.  A third alternative is @code{%nonassoc}, which
+declares that it is a syntax error to find the same operator twice ``in a
+row''.
+
+The relative precedence of different operators is controlled by the
+order in which they are declared.  The first @code{%left} or
+@code{%right} declaration in the file declares the operators whose
+precedence is lowest, the next such declaration declares the operators
+whose precedence is a little higher, and so on.
+
+@node Precedence Examples
+@subsection Precedence Examples
+
+In our example, we would want the following declarations:
+
+@example
+%left '<'
+%left '-'
+%left '*'
+@end example
+
+In a more complete example, which supports other operators as well, we
+would declare them in groups of equal precedence.  For example, @code{'+'} is
+declared with @code{'-'}:
+
+@example
+%left '<' '>' '=' NE LE GE
+%left '+' '-'
+%left '*' '/'
+@end example
+
+@noindent
+(Here @code{NE} and so on stand for the operators for ``not equal''
+and so on.  We assume that these tokens are more than one character long
+and therefore are represented by names, not character literals.)
+
+@node How Precedence
+@subsection How Precedence Works
+
+The first effect of the precedence declarations is to assign precedence
+levels to the terminal symbols declared.  The second effect is to assign
+precedence levels to certain rules: each rule gets its precedence from
+the last terminal symbol mentioned in the components.  (You can also
+specify explicitly the precedence of a rule.  @xref{Contextual
+Precedence, ,Context-Dependent Precedence}.)
+
+Finally, the resolution of conflicts works by comparing the precedence
+of the rule being considered with that of the lookahead token.  If the
+token's precedence is higher, the choice is to shift.  If the rule's
+precedence is higher, the choice is to reduce.  If they have equal
+precedence, the choice is made based on the associativity of that
+precedence level.  The verbose output file made by @samp{-v}
+(@pxref{Invocation, ,Invoking Bison}) says how each conflict was
+resolved.
+
+Not all rules and not all tokens have precedence.  If either the rule or
+the lookahead token has no precedence, then the default is to shift.
+
+@node Contextual Precedence
+@section Context-Dependent Precedence
+@cindex context-dependent precedence
+@cindex unary operator precedence
+@cindex precedence, context-dependent
+@cindex precedence, unary operator
+@findex %prec
+
+Often the precedence of an operator depends on the context.  This sounds
+outlandish at first, but it is really very common.  For example, a minus
+sign typically has a very high precedence as a unary operator, and a
+somewhat lower precedence (lower than multiplication) as a binary operator.
+
+The Bison precedence declarations, @code{%left}, @code{%right} and
+@code{%nonassoc}, can only be used once for a given token; so a token has
+only one precedence declared in this way.  For context-dependent
+precedence, you need to use an additional mechanism: the @code{%prec}
+modifier for rules.
+
+The @code{%prec} modifier declares the precedence of a particular rule by
+specifying a terminal symbol whose precedence should be used for that rule.
+It's not necessary for that symbol to appear otherwise in the rule.  The
+modifier's syntax is:
+
+@example
+%prec @var{terminal-symbol}
+@end example
+
+@noindent
+and it is written after the components of the rule.  Its effect is to
+assign the rule the precedence of @var{terminal-symbol}, overriding
+the precedence that would be deduced for it in the ordinary way.  The
+altered rule precedence then affects how conflicts involving that rule
+are resolved (@pxref{Precedence, ,Operator Precedence}).
+
+Here is how @code{%prec} solves the problem of unary minus.  First, declare
+a precedence for a fictitious terminal symbol named @code{UMINUS}.  There
+are no tokens of this type, but the symbol serves to stand for its
+precedence:
+
+@example
+@dots{}
+%left '+' '-'
+%left '*'
+%left UMINUS
+@end example
+
+Now the precedence of @code{UMINUS} can be used in specific rules:
+
+@example
+@group
+exp:    @dots{}
+        | exp '-' exp
+        @dots{}
+        | '-' exp %prec UMINUS
+@end group
+@end example
+
+@ifset defaultprec
+If you forget to append @code{%prec UMINUS} to the rule for unary
+minus, Bison silently assumes that minus has its usual precedence.
+This kind of problem can be tricky to debug, since one typically
+discovers the mistake only by testing the code.
+
+The @code{%no-default-prec;} declaration makes it easier to discover
+this kind of problem systematically.  It causes rules that lack a
+@code{%prec} modifier to have no precedence, even if the last terminal
+symbol mentioned in their components has a declared precedence.
+
+If @code{%no-default-prec;} is in effect, you must specify @code{%prec}
+for all rules that participate in precedence conflict resolution.
+Then you will see any shift/reduce conflict until you tell Bison how
+to resolve it, either by changing your grammar or by adding an
+explicit precedence.  This will probably add declarations to the
+grammar, but it helps to protect against incorrect rule precedences.
+
+The effect of @code{%no-default-prec;} can be reversed by giving
+@code{%default-prec;}, which is the default.
+@end ifset
+
+@node Parser States
+@section Parser States
+@cindex finite-state machine
+@cindex parser state
+@cindex state (of parser)
+
+The function @code{yyparse} is implemented using a finite-state machine.
+The values pushed on the parser stack are not simply token type codes; they
+represent the entire sequence of terminal and nonterminal symbols at or
+near the top of the stack.  The current state collects all the information
+about previous input which is relevant to deciding what to do next.
+
+Each time a lookahead token is read, the current parser state together
+with the type of lookahead token are looked up in a table.  This table
+entry can say, ``Shift the lookahead token.''  In this case, it also
+specifies the new parser state, which is pushed onto the top of the
+parser stack.  Or it can say, ``Reduce using rule number @var{n}.''
+This means that a certain number of tokens or groupings are taken off
+the top of the stack, and replaced by one grouping.  In other words,
+that number of states are popped from the stack, and one new state is
+pushed.
+
+There is one other alternative: the table can say that the lookahead token
+is erroneous in the current state.  This causes error processing to begin
+(@pxref{Error Recovery}).
+
+@node Reduce/Reduce
+@section Reduce/Reduce Conflicts
+@cindex reduce/reduce conflict
+@cindex conflicts, reduce/reduce
+
+A reduce/reduce conflict occurs if there are two or more rules that apply
+to the same sequence of input.  This usually indicates a serious error
+in the grammar.
+
+For example, here is an erroneous attempt to define a sequence
+of zero or more @code{word} groupings.
+
+@example
+sequence: /* empty */
+                @{ printf ("empty sequence\n"); @}
+        | maybeword
+        | sequence word
+                @{ printf ("added word %s\n", $2); @}
+        ;
+
+maybeword: /* empty */
+                @{ printf ("empty maybeword\n"); @}
+        | word
+                @{ printf ("single word %s\n", $1); @}
+        ;
+@end example
+
+@noindent
+The error is an ambiguity: there is more than one way to parse a single
+@code{word} into a @code{sequence}.  It could be reduced to a
+@code{maybeword} and then into a @code{sequence} via the second rule.
+Alternatively, nothing-at-all could be reduced into a @code{sequence}
+via the first rule, and this could be combined with the @code{word}
+using the third rule for @code{sequence}.
+
+There is also more than one way to reduce nothing-at-all into a
+@code{sequence}.  This can be done directly via the first rule,
+or indirectly via @code{maybeword} and then the second rule.
+
+You might think that this is a distinction without a difference, because it
+does not change whether any particular input is valid or not.  But it does
+affect which actions are run.  One parsing order runs the second rule's
+action; the other runs the first rule's action and the third rule's action.
+In this example, the output of the program changes.
+
+Bison resolves a reduce/reduce conflict by choosing to use the rule that
+appears first in the grammar, but it is very risky to rely on this.  Every
+reduce/reduce conflict must be studied and usually eliminated.  Here is the
+proper way to define @code{sequence}:
+
+@example
+sequence: /* empty */
+                @{ printf ("empty sequence\n"); @}
+        | sequence word
+                @{ printf ("added word %s\n", $2); @}
+        ;
+@end example
+
+Here is another common error that yields a reduce/reduce conflict:
+
+@example
+sequence: /* empty */
+        | sequence words
+        | sequence redirects
+        ;
+
+words:    /* empty */
+        | words word
+        ;
+
+redirects:/* empty */
+        | redirects redirect
+        ;
+@end example
+
+@noindent
+The intention here is to define a sequence which can contain either
+@code{word} or @code{redirect} groupings.  The individual definitions of
+@code{sequence}, @code{words} and @code{redirects} are error-free, but the
+three together make a subtle ambiguity: even an empty input can be parsed
+in infinitely many ways!
+
+Consider: nothing-at-all could be a @code{words}.  Or it could be two
+@code{words} in a row, or three, or any number.  It could equally well be a
+@code{redirects}, or two, or any number.  Or it could be a @code{words}
+followed by three @code{redirects} and another @code{words}.  And so on.
+
+Here are two ways to correct these rules.  First, to make it a single level
+of sequence:
+
+@example
+sequence: /* empty */
+        | sequence word
+        | sequence redirect
+        ;
+@end example
+
+Second, to prevent either a @code{words} or a @code{redirects}
+from being empty:
+
+@example
+sequence: /* empty */
+        | sequence words
+        | sequence redirects
+        ;
+
+words:    word
+        | words word
+        ;
+
+redirects:redirect
+        | redirects redirect
+        ;
+@end example
+
+@node Mystery Conflicts
+@section Mysterious Reduce/Reduce Conflicts
+
+Sometimes reduce/reduce conflicts can occur that don't look warranted.
+Here is an example:
+
+@example
+@group
+%token ID
+
+%%
+def:    param_spec return_spec ','
+        ;
+param_spec:
+             type
+        |    name_list ':' type
+        ;
+@end group
+@group
+return_spec:
+             type
+        |    name ':' type
+        ;
+@end group
+@group
+type:        ID
+        ;
+@end group
+@group
+name:        ID
+        ;
+name_list:
+             name
+        |    name ',' name_list
+        ;
+@end group
+@end example
+
+It would seem that this grammar can be parsed with only a single token
+of lookahead: when a @code{param_spec} is being read, an @code{ID} is
+a @code{name} if a comma or colon follows, or a @code{type} if another
+@code{ID} follows.  In other words, this grammar is @acronym{LR}(1).
+
+@cindex @acronym{LR}(1)
+@cindex @acronym{LALR}(1)
+However, Bison, like most parser generators, cannot actually handle all
+@acronym{LR}(1) grammars.  In this grammar, two contexts, that after
+an @code{ID}
+at the beginning of a @code{param_spec} and likewise at the beginning of
+a @code{return_spec}, are similar enough that Bison assumes they are the
+same.  They appear similar because the same set of rules would be
+active---the rule for reducing to a @code{name} and that for reducing to
+a @code{type}.  Bison is unable to determine at that stage of processing
+that the rules would require different lookahead tokens in the two
+contexts, so it makes a single parser state for them both.  Combining
+the two contexts causes a conflict later.  In parser terminology, this
+occurrence means that the grammar is not @acronym{LALR}(1).
+
+In general, it is better to fix deficiencies than to document them.  But
+this particular deficiency is intrinsically hard to fix; parser
+generators that can handle @acronym{LR}(1) grammars are hard to write
+and tend to
+produce parsers that are very large.  In practice, Bison is more useful
+as it is now.
+
+When the problem arises, you can often fix it by identifying the two
+parser states that are being confused, and adding something to make them
+look distinct.  In the above example, adding one rule to
+@code{return_spec} as follows makes the problem go away:
+
+@example
+@group
+%token BOGUS
+@dots{}
+%%
+@dots{}
+return_spec:
+             type
+        |    name ':' type
+        /* This rule is never used.  */
+        |    ID BOGUS
+        ;
+@end group
+@end example
+
+This corrects the problem because it introduces the possibility of an
+additional active rule in the context after the @code{ID} at the beginning of
+@code{return_spec}.  This rule is not active in the corresponding context
+in a @code{param_spec}, so the two contexts receive distinct parser states.
+As long as the token @code{BOGUS} is never generated by @code{yylex},
+the added rule cannot alter the way actual input is parsed.
+
+In this particular example, there is another way to solve the problem:
+rewrite the rule for @code{return_spec} to use @code{ID} directly
+instead of via @code{name}.  This also causes the two confusing
+contexts to have different sets of active rules, because the one for
+@code{return_spec} activates the altered rule for @code{return_spec}
+rather than the one for @code{name}.
+
+@example
+param_spec:
+             type
+        |    name_list ':' type
+        ;
+return_spec:
+             type
+        |    ID ':' type
+        ;
+@end example
+
+For a more detailed exposition of @acronym{LALR}(1) parsers and parser
+generators, please see:
+Frank DeRemer and Thomas Pennello, Efficient Computation of
+@acronym{LALR}(1) Look-Ahead Sets, @cite{@acronym{ACM} Transactions on
+Programming Languages and Systems}, Vol.@: 4, No.@: 4 (October 1982),
+pp.@: 615--649 @uref{http://doi.acm.org/10.1145/69622.357187}.
+
+@node Generalized LR Parsing
+@section Generalized @acronym{LR} (@acronym{GLR}) Parsing
+@cindex @acronym{GLR} parsing
+@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
+@cindex ambiguous grammars
+@cindex nondeterministic parsing
+
+Bison produces @emph{deterministic} parsers that choose uniquely
+when to reduce and which reduction to apply
+based on a summary of the preceding input and on one extra token of lookahead.
+As a result, normal Bison handles a proper subset of the family of
+context-free languages.
+Ambiguous grammars, since they have strings with more than one possible
+sequence of reductions cannot have deterministic parsers in this sense.
+The same is true of languages that require more than one symbol of
+lookahead, since the parser lacks the information necessary to make a
+decision at the point it must be made in a shift-reduce parser.
+Finally, as previously mentioned (@pxref{Mystery Conflicts}),
+there are languages where Bison's particular choice of how to
+summarize the input seen so far loses necessary information.
+
+When you use the @samp{%glr-parser} declaration in your grammar file,
+Bison generates a parser that uses a different algorithm, called
+Generalized @acronym{LR} (or @acronym{GLR}).  A Bison @acronym{GLR}
+parser uses the same basic
+algorithm for parsing as an ordinary Bison parser, but behaves
+differently in cases where there is a shift-reduce conflict that has not
+been resolved by precedence rules (@pxref{Precedence}) or a
+reduce-reduce conflict.  When a @acronym{GLR} parser encounters such a
+situation, it
+effectively @emph{splits} into a several parsers, one for each possible
+shift or reduction.  These parsers then proceed as usual, consuming
+tokens in lock-step.  Some of the stacks may encounter other conflicts
+and split further, with the result that instead of a sequence of states,
+a Bison @acronym{GLR} parsing stack is what is in effect a tree of states.
+
+In effect, each stack represents a guess as to what the proper parse
+is.  Additional input may indicate that a guess was wrong, in which case
+the appropriate stack silently disappears.  Otherwise, the semantics
+actions generated in each stack are saved, rather than being executed
+immediately.  When a stack disappears, its saved semantic actions never
+get executed.  When a reduction causes two stacks to become equivalent,
+their sets of semantic actions are both saved with the state that
+results from the reduction.  We say that two stacks are equivalent
+when they both represent the same sequence of states,
+and each pair of corresponding states represents a
+grammar symbol that produces the same segment of the input token
+stream.
+
+Whenever the parser makes a transition from having multiple
+states to having one, it reverts to the normal @acronym{LALR}(1) parsing
+algorithm, after resolving and executing the saved-up actions.
+At this transition, some of the states on the stack will have semantic
+values that are sets (actually multisets) of possible actions.  The
+parser tries to pick one of the actions by first finding one whose rule
+has the highest dynamic precedence, as set by the @samp{%dprec}
+declaration.  Otherwise, if the alternative actions are not ordered by
+precedence, but there the same merging function is declared for both
+rules by the @samp{%merge} declaration,
+Bison resolves and evaluates both and then calls the merge function on
+the result.  Otherwise, it reports an ambiguity.
+
+It is possible to use a data structure for the @acronym{GLR} parsing tree that
+permits the processing of any @acronym{LALR}(1) grammar in linear time (in the
+size of the input), any unambiguous (not necessarily
+@acronym{LALR}(1)) grammar in
+quadratic worst-case time, and any general (possibly ambiguous)
+context-free grammar in cubic worst-case time.  However, Bison currently
+uses a simpler data structure that requires time proportional to the
+length of the input times the maximum number of stacks required for any
+prefix of the input.  Thus, really ambiguous or nondeterministic
+grammars can require exponential time and space to process.  Such badly
+behaving examples, however, are not generally of practical interest.
+Usually, nondeterminism in a grammar is local---the parser is ``in
+doubt'' only for a few tokens at a time.  Therefore, the current data
+structure should generally be adequate.  On @acronym{LALR}(1) portions of a
+grammar, in particular, it is only slightly slower than with the default
+Bison parser.
+
+For a more detailed exposition of @acronym{GLR} parsers, please see: Elizabeth
+Scott, Adrian Johnstone and Shamsa Sadaf Hussain, Tomita-Style
+Generalised @acronym{LR} Parsers, Royal Holloway, University of
+London, Department of Computer Science, TR-00-12,
+@uref{http://www.cs.rhul.ac.uk/research/languages/publications/tomita_style_1.ps},
+(2000-12-24).
+
+@node Memory Management
+@section Memory Management, and How to Avoid Memory Exhaustion
+@cindex memory exhaustion
+@cindex memory management
+@cindex stack overflow
+@cindex parser stack overflow
+@cindex overflow of parser stack
+
+The Bison parser stack can run out of memory if too many tokens are shifted and
+not reduced.  When this happens, the parser function @code{yyparse}
+calls @code{yyerror} and then returns 2.
+
+Because Bison parsers have growing stacks, hitting the upper limit
+usually results from using a right recursion instead of a left
+recursion, @xref{Recursion, ,Recursive Rules}.
+
+@vindex YYMAXDEPTH
+By defining the macro @code{YYMAXDEPTH}, you can control how deep the
+parser stack can become before memory is exhausted.  Define the
+macro with a value that is an integer.  This value is the maximum number
+of tokens that can be shifted (and not reduced) before overflow.
+
+The stack space allowed is not necessarily allocated.  If you specify a
+large value for @code{YYMAXDEPTH}, the parser normally allocates a small
+stack at first, and then makes it bigger by stages as needed.  This
+increasing allocation happens automatically and silently.  Therefore,
+you do not need to make @code{YYMAXDEPTH} painfully small merely to save
+space for ordinary inputs that do not need much stack.
+
+However, do not allow @code{YYMAXDEPTH} to be a value so large that
+arithmetic overflow could occur when calculating the size of the stack
+space.  Also, do not allow @code{YYMAXDEPTH} to be less than
+@code{YYINITDEPTH}.
+
+@cindex default stack limit
+The default value of @code{YYMAXDEPTH}, if you do not define it, is
+10000.
+
+@vindex YYINITDEPTH
+You can control how much stack is allocated initially by defining the
+macro @code{YYINITDEPTH} to a positive integer.  For the C
+@acronym{LALR}(1) parser, this value must be a compile-time constant
+unless you are assuming C99 or some other target language or compiler
+that allows variable-length arrays.  The default is 200.
+
+Do not allow @code{YYINITDEPTH} to be greater than @code{YYMAXDEPTH}.
+
+@c FIXME: C++ output.
+Because of semantical differences between C and C++, the
+@acronym{LALR}(1) parsers in C produced by Bison cannot grow when compiled
+by C++ compilers.  In this precise case (compiling a C parser as C++) you are
+suggested to grow @code{YYINITDEPTH}.  The Bison maintainers hope to fix
+this deficiency in a future release.
+
+@node Error Recovery
+@chapter Error Recovery
+@cindex error recovery
+@cindex recovery from errors
+
+It is not usually acceptable to have a program terminate on a syntax
+error.  For example, a compiler should recover sufficiently to parse the
+rest of the input file and check it for errors; a calculator should accept
+another expression.
+
+In a simple interactive command parser where each input is one line, it may
+be sufficient to allow @code{yyparse} to return 1 on error and have the
+caller ignore the rest of the input line when that happens (and then call
+@code{yyparse} again).  But this is inadequate for a compiler, because it
+forgets all the syntactic context leading up to the error.  A syntax error
+deep within a function in the compiler input should not cause the compiler
+to treat the following line like the beginning of a source file.
+
+@findex error
+You can define how to recover from a syntax error by writing rules to
+recognize the special token @code{error}.  This is a terminal symbol that
+is always defined (you need not declare it) and reserved for error
+handling.  The Bison parser generates an @code{error} token whenever a
+syntax error happens; if you have provided a rule to recognize this token
+in the current context, the parse can continue.
+
+For example:
+
+@example
+stmnts:  /* empty string */
+        | stmnts '\n'
+        | stmnts exp '\n'
+        | stmnts error '\n'
+@end example
+
+The fourth rule in this example says that an error followed by a newline
+makes a valid addition to any @code{stmnts}.
+
+What happens if a syntax error occurs in the middle of an @code{exp}?  The
+error recovery rule, interpreted strictly, applies to the precise sequence
+of a @code{stmnts}, an @code{error} and a newline.  If an error occurs in
+the middle of an @code{exp}, there will probably be some additional tokens
+and subexpressions on the stack after the last @code{stmnts}, and there
+will be tokens to read before the next newline.  So the rule is not
+applicable in the ordinary way.
+
+But Bison can force the situation to fit the rule, by discarding part of
+the semantic context and part of the input.  First it discards states
+and objects from the stack until it gets back to a state in which the
+@code{error} token is acceptable.  (This means that the subexpressions
+already parsed are discarded, back to the last complete @code{stmnts}.)
+At this point the @code{error} token can be shifted.  Then, if the old
+lookahead token is not acceptable to be shifted next, the parser reads
+tokens and discards them until it finds a token which is acceptable.  In
+this example, Bison reads and discards input until the next newline so
+that the fourth rule can apply.  Note that discarded symbols are
+possible sources of memory leaks, see @ref{Destructor Decl, , Freeing
+Discarded Symbols}, for a means to reclaim this memory.
+
+The choice of error rules in the grammar is a choice of strategies for
+error recovery.  A simple and useful strategy is simply to skip the rest of
+the current input line or current statement if an error is detected:
+
+@example
+stmnt: error ';'  /* On error, skip until ';' is read.  */
+@end example
+
+It is also useful to recover to the matching close-delimiter of an
+opening-delimiter that has already been parsed.  Otherwise the
+close-delimiter will probably appear to be unmatched, and generate another,
+spurious error message:
+
+@example
+primary:  '(' expr ')'
+        | '(' error ')'
+        @dots{}
+        ;
+@end example
+
+Error recovery strategies are necessarily guesses.  When they guess wrong,
+one syntax error often leads to another.  In the above example, the error
+recovery rule guesses that an error is due to bad input within one
+@code{stmnt}.  Suppose that instead a spurious semicolon is inserted in the
+middle of a valid @code{stmnt}.  After the error recovery rule recovers
+from the first error, another syntax error will be found straightaway,
+since the text following the spurious semicolon is also an invalid
+@code{stmnt}.
+
+To prevent an outpouring of error messages, the parser will output no error
+message for another syntax error that happens shortly after the first; only
+after three consecutive input tokens have been successfully shifted will
+error messages resume.
+
+Note that rules which accept the @code{error} token may have actions, just
+as any other rules can.
+
+@findex yyerrok
+You can make error messages resume immediately by using the macro
+@code{yyerrok} in an action.  If you do this in the error rule's action, no
+error messages will be suppressed.  This macro requires no arguments;
+@samp{yyerrok;} is a valid C statement.
+
+@findex yyclearin
+The previous lookahead token is reanalyzed immediately after an error.  If
+this is unacceptable, then the macro @code{yyclearin} may be used to clear
+this token.  Write the statement @samp{yyclearin;} in the error rule's
+action.
+@xref{Action Features, ,Special Features for Use in Actions}.
+
+For example, suppose that on a syntax error, an error handling routine is
+called that advances the input stream to some point where parsing should
+once again commence.  The next symbol returned by the lexical scanner is
+probably correct.  The previous lookahead token ought to be discarded
+with @samp{yyclearin;}.
+
+@vindex YYRECOVERING
+The expression @code{YYRECOVERING ()} yields 1 when the parser
+is recovering from a syntax error, and 0 otherwise.
+Syntax error diagnostics are suppressed while recovering from a syntax
+error.
+
+@node Context Dependency
+@chapter Handling Context Dependencies
+
+The Bison paradigm is to parse tokens first, then group them into larger
+syntactic units.  In many languages, the meaning of a token is affected by
+its context.  Although this violates the Bison paradigm, certain techniques
+(known as @dfn{kludges}) may enable you to write Bison parsers for such
+languages.
+
+@menu
+* Semantic Tokens::   Token parsing can depend on the semantic context.
+* Lexical Tie-ins::   Token parsing can depend on the syntactic context.
+* Tie-in Recovery::   Lexical tie-ins have implications for how
+                        error recovery rules must be written.
+@end menu
+
+(Actually, ``kludge'' means any technique that gets its job done but is
+neither clean nor robust.)
+
+@node Semantic Tokens
+@section Semantic Info in Token Types
+
+The C language has a context dependency: the way an identifier is used
+depends on what its current meaning is.  For example, consider this:
+
+@example
+foo (x);
+@end example
+
+This looks like a function call statement, but if @code{foo} is a typedef
+name, then this is actually a declaration of @code{x}.  How can a Bison
+parser for C decide how to parse this input?
+
+The method used in @acronym{GNU} C is to have two different token types,
+@code{IDENTIFIER} and @code{TYPENAME}.  When @code{yylex} finds an
+identifier, it looks up the current declaration of the identifier in order
+to decide which token type to return: @code{TYPENAME} if the identifier is
+declared as a typedef, @code{IDENTIFIER} otherwise.
+
+The grammar rules can then express the context dependency by the choice of
+token type to recognize.  @code{IDENTIFIER} is accepted as an expression,
+but @code{TYPENAME} is not.  @code{TYPENAME} can start a declaration, but
+@code{IDENTIFIER} cannot.  In contexts where the meaning of the identifier
+is @emph{not} significant, such as in declarations that can shadow a
+typedef name, either @code{TYPENAME} or @code{IDENTIFIER} is
+accepted---there is one rule for each of the two token types.
+
+This technique is simple to use if the decision of which kinds of
+identifiers to allow is made at a place close to where the identifier is
+parsed.  But in C this is not always so: C allows a declaration to
+redeclare a typedef name provided an explicit type has been specified
+earlier:
+
+@example
+typedef int foo, bar;
+int baz (void)
+@{
+  static bar (bar);      /* @r{redeclare @code{bar} as static variable} */
+  extern foo foo (foo);  /* @r{redeclare @code{foo} as function} */
+  return foo (bar);
+@}
+@end example
+
+Unfortunately, the name being declared is separated from the declaration
+construct itself by a complicated syntactic structure---the ``declarator''.
+
+As a result, part of the Bison parser for C needs to be duplicated, with
+all the nonterminal names changed: once for parsing a declaration in
+which a typedef name can be redefined, and once for parsing a
+declaration in which that can't be done.  Here is a part of the
+duplication, with actions omitted for brevity:
+
+@example
+initdcl:
+          declarator maybeasm '='
+          init
+        | declarator maybeasm
+        ;
+
+notype_initdcl:
+          notype_declarator maybeasm '='
+          init
+        | notype_declarator maybeasm
+        ;
+@end example
+
+@noindent
+Here @code{initdcl} can redeclare a typedef name, but @code{notype_initdcl}
+cannot.  The distinction between @code{declarator} and
+@code{notype_declarator} is the same sort of thing.
+
+There is some similarity between this technique and a lexical tie-in
+(described next), in that information which alters the lexical analysis is
+changed during parsing by other parts of the program.  The difference is
+here the information is global, and is used for other purposes in the
+program.  A true lexical tie-in has a special-purpose flag controlled by
+the syntactic context.
+
+@node Lexical Tie-ins
+@section Lexical Tie-ins
+@cindex lexical tie-in
+
+One way to handle context-dependency is the @dfn{lexical tie-in}: a flag
+which is set by Bison actions, whose purpose is to alter the way tokens are
+parsed.
+
+For example, suppose we have a language vaguely like C, but with a special
+construct @samp{hex (@var{hex-expr})}.  After the keyword @code{hex} comes
+an expression in parentheses in which all integers are hexadecimal.  In
+particular, the token @samp{a1b} must be treated as an integer rather than
+as an identifier if it appears in that context.  Here is how you can do it:
+
+@example
+@group
+%@{
+  int hexflag;
+  int yylex (void);
+  void yyerror (char const *);
+%@}
+%%
+@dots{}
+@end group
+@group
+expr:   IDENTIFIER
+        | constant
+        | HEX '('
+                @{ hexflag = 1; @}
+          expr ')'
+                @{ hexflag = 0;
+                   $$ = $4; @}
+        | expr '+' expr
+                @{ $$ = make_sum ($1, $3); @}
+        @dots{}
+        ;
+@end group
+
+@group
+constant:
+          INTEGER
+        | STRING
+        ;
+@end group
+@end example
+
+@noindent
+Here we assume that @code{yylex} looks at the value of @code{hexflag}; when
+it is nonzero, all integers are parsed in hexadecimal, and tokens starting
+with letters are parsed as integers if possible.
+
+The declaration of @code{hexflag} shown in the prologue of the parser file
+is needed to make it accessible to the actions (@pxref{Prologue, ,The Prologue}).
+You must also write the code in @code{yylex} to obey the flag.
+
+@node Tie-in Recovery
+@section Lexical Tie-ins and Error Recovery
+
+Lexical tie-ins make strict demands on any error recovery rules you have.
+@xref{Error Recovery}.
+
+The reason for this is that the purpose of an error recovery rule is to
+abort the parsing of one construct and resume in some larger construct.
+For example, in C-like languages, a typical error recovery rule is to skip
+tokens until the next semicolon, and then start a new statement, like this:
+
+@example
+stmt:   expr ';'
+        | IF '(' expr ')' stmt @{ @dots{} @}
+        @dots{}
+        error ';'
+                @{ hexflag = 0; @}
+        ;
+@end example
+
+If there is a syntax error in the middle of a @samp{hex (@var{expr})}
+construct, this error rule will apply, and then the action for the
+completed @samp{hex (@var{expr})} will never run.  So @code{hexflag} would
+remain set for the entire rest of the input, or until the next @code{hex}
+keyword, causing identifiers to be misinterpreted as integers.
+
+To avoid this problem the error recovery rule itself clears @code{hexflag}.
+
+There may also be an error recovery rule that works within expressions.
+For example, there could be a rule which applies within parentheses
+and skips to the close-parenthesis:
+
+@example
+@group
+expr:   @dots{}
+        | '(' expr ')'
+                @{ $$ = $2; @}
+        | '(' error ')'
+        @dots{}
+@end group
+@end example
+
+If this rule acts within the @code{hex} construct, it is not going to abort
+that construct (since it applies to an inner level of parentheses within
+the construct).  Therefore, it should not clear the flag: the rest of
+the @code{hex} construct should be parsed with the flag still in effect.
+
+What if there is an error recovery rule which might abort out of the
+@code{hex} construct or might not, depending on circumstances?  There is no
+way you can write the action to determine whether a @code{hex} construct is
+being aborted or not.  So if you are using a lexical tie-in, you had better
+make sure your error recovery rules are not of this kind.  Each rule must
+be such that you can be sure that it always will, or always won't, have to
+clear the flag.
+
+@c ================================================== Debugging Your Parser
+
+@node Debugging
+@chapter Debugging Your Parser
+
+Developing a parser can be a challenge, especially if you don't
+understand the algorithm (@pxref{Algorithm, ,The Bison Parser
+Algorithm}).  Even so, sometimes a detailed description of the automaton
+can help (@pxref{Understanding, , Understanding Your Parser}), or
+tracing the execution of the parser can give some insight on why it
+behaves improperly (@pxref{Tracing, , Tracing Your Parser}).
+
+@menu
+* Understanding::     Understanding the structure of your parser.
+* Tracing::           Tracing the execution of your parser.
+@end menu
+
+@node Understanding
+@section Understanding Your Parser
+
+As documented elsewhere (@pxref{Algorithm, ,The Bison Parser Algorithm})
+Bison parsers are @dfn{shift/reduce automata}.  In some cases (much more
+frequent than one would hope), looking at this automaton is required to
+tune or simply fix a parser.  Bison provides two different
+representation of it, either textually or graphically (as a DOT file).
+
+The textual file is generated when the options @option{--report} or
+@option{--verbose} are specified, see @xref{Invocation, , Invoking
+Bison}.  Its name is made by removing @samp{.tab.c} or @samp{.c} from
+the parser output file name, and adding @samp{.output} instead.
+Therefore, if the input file is @file{foo.y}, then the parser file is
+called @file{foo.tab.c} by default.  As a consequence, the verbose
+output file is called @file{foo.output}.
+
+The following grammar file, @file{calc.y}, will be used in the sequel:
+
+@example
+%token NUM STR
+%left '+' '-'
+%left '*'
+%%
+exp: exp '+' exp
+   | exp '-' exp
+   | exp '*' exp
+   | exp '/' exp
+   | NUM
+   ;
+useless: STR;
+%%
+@end example
+
+@command{bison} reports:
+
+@example
+calc.y: warning: 1 nonterminal and 1 rule useless in grammar
+calc.y:11.1-7: warning: nonterminal useless in grammar: useless
+calc.y:11.10-12: warning: rule useless in grammar: useless: STR
+calc.y: conflicts: 7 shift/reduce
+@end example
+
+When given @option{--report=state}, in addition to @file{calc.tab.c}, it
+creates a file @file{calc.output} with contents detailed below.  The
+order of the output and the exact presentation might vary, but the
+interpretation is the same.
+
+The first section includes details on conflicts that were solved thanks
+to precedence and/or associativity:
+
+@example
+Conflict in state 8 between rule 2 and token '+' resolved as reduce.
+Conflict in state 8 between rule 2 and token '-' resolved as reduce.
+Conflict in state 8 between rule 2 and token '*' resolved as shift.
+@exdent @dots{}
+@end example
+
+@noindent
+The next section lists states that still have conflicts.
+
+@example
+State 8 conflicts: 1 shift/reduce
+State 9 conflicts: 1 shift/reduce
+State 10 conflicts: 1 shift/reduce
+State 11 conflicts: 4 shift/reduce
+@end example
+
+@noindent
+@cindex token, useless
+@cindex useless token
+@cindex nonterminal, useless
+@cindex useless nonterminal
+@cindex rule, useless
+@cindex useless rule
+The next section reports useless tokens, nonterminal and rules.  Useless
+nonterminals and rules are removed in order to produce a smaller parser,
+but useless tokens are preserved, since they might be used by the
+scanner (note the difference between ``useless'' and ``unused''
+below):
+
+@example
+Nonterminals useless in grammar:
+   useless
+
+Terminals unused in grammar:
+   STR
+
+Rules useless in grammar:
+#6     useless: STR;
+@end example
+
+@noindent
+The next section reproduces the exact grammar that Bison used:
+
+@example
+Grammar
+
+  Number, Line, Rule
+    0   5 $accept -> exp $end
+    1   5 exp -> exp '+' exp
+    2   6 exp -> exp '-' exp
+    3   7 exp -> exp '*' exp
+    4   8 exp -> exp '/' exp
+    5   9 exp -> NUM
+@end example
+
+@noindent
+and reports the uses of the symbols:
+
+@example
+Terminals, with rules where they appear
+
+$end (0) 0
+'*' (42) 3
+'+' (43) 1
+'-' (45) 2
+'/' (47) 4
+error (256)
+NUM (258) 5
+
+Nonterminals, with rules where they appear
+
+$accept (8)
+    on left: 0
+exp (9)
+    on left: 1 2 3 4 5, on right: 0 1 2 3 4
+@end example
+
+@noindent
+@cindex item
+@cindex pointed rule
+@cindex rule, pointed
+Bison then proceeds onto the automaton itself, describing each state
+with it set of @dfn{items}, also known as @dfn{pointed rules}.  Each
+item is a production rule together with a point (marked by @samp{.})
+that the input cursor.
+
+@example
+state 0
+
+    $accept  ->  . exp $   (rule 0)
+
+    NUM         shift, and go to state 1
+
+    exp         go to state 2
+@end example
+
+This reads as follows: ``state 0 corresponds to being at the very
+beginning of the parsing, in the initial rule, right before the start
+symbol (here, @code{exp}).  When the parser returns to this state right
+after having reduced a rule that produced an @code{exp}, the control
+flow jumps to state 2.  If there is no such transition on a nonterminal
+symbol, and the lookahead is a @code{NUM}, then this token is shifted on
+the parse stack, and the control flow jumps to state 1.  Any other
+lookahead triggers a syntax error.''
+
+@cindex core, item set
+@cindex item set core
+@cindex kernel, item set
+@cindex item set core
+Even though the only active rule in state 0 seems to be rule 0, the
+report lists @code{NUM} as a lookahead token because @code{NUM} can be
+at the beginning of any rule deriving an @code{exp}.  By default Bison
+reports the so-called @dfn{core} or @dfn{kernel} of the item set, but if
+you want to see more detail you can invoke @command{bison} with
+@option{--report=itemset} to list all the items, include those that can
+be derived:
+
+@example
+state 0
+
+    $accept  ->  . exp $   (rule 0)
+    exp  ->  . exp '+' exp   (rule 1)
+    exp  ->  . exp '-' exp   (rule 2)
+    exp  ->  . exp '*' exp   (rule 3)
+    exp  ->  . exp '/' exp   (rule 4)
+    exp  ->  . NUM   (rule 5)
+
+    NUM         shift, and go to state 1
+
+    exp         go to state 2
+@end example
+
+@noindent
+In the state 1...
+
+@example
+state 1
+
+    exp  ->  NUM .   (rule 5)
+
+    $default    reduce using rule 5 (exp)
+@end example
+
+@noindent
+the rule 5, @samp{exp: NUM;}, is completed.  Whatever the lookahead token
+(@samp{$default}), the parser will reduce it.  If it was coming from
+state 0, then, after this reduction it will return to state 0, and will
+jump to state 2 (@samp{exp: go to state 2}).
+
+@example
+state 2
+
+    $accept  ->  exp . $   (rule 0)
+    exp  ->  exp . '+' exp   (rule 1)
+    exp  ->  exp . '-' exp   (rule 2)
+    exp  ->  exp . '*' exp   (rule 3)
+    exp  ->  exp . '/' exp   (rule 4)
+
+    $           shift, and go to state 3
+    '+'         shift, and go to state 4
+    '-'         shift, and go to state 5
+    '*'         shift, and go to state 6
+    '/'         shift, and go to state 7
+@end example
+
+@noindent
+In state 2, the automaton can only shift a symbol.  For instance,
+because of the item @samp{exp -> exp . '+' exp}, if the lookahead if
+@samp{+}, it will be shifted on the parse stack, and the automaton
+control will jump to state 4, corresponding to the item @samp{exp -> exp
+'+' . exp}.  Since there is no default action, any other token than
+those listed above will trigger a syntax error.
+
+The state 3 is named the @dfn{final state}, or the @dfn{accepting
+state}:
+
+@example
+state 3
+
+    $accept  ->  exp $ .   (rule 0)
+
+    $default    accept
+@end example
+
+@noindent
+the initial rule is completed (the start symbol and the end
+of input were read), the parsing exits successfully.
+
+The interpretation of states 4 to 7 is straightforward, and is left to
+the reader.
+
+@example
+state 4
+
+    exp  ->  exp '+' . exp   (rule 1)
+
+    NUM         shift, and go to state 1
+
+    exp         go to state 8
+
+state 5
+
+    exp  ->  exp '-' . exp   (rule 2)
+
+    NUM         shift, and go to state 1
+
+    exp         go to state 9
+
+state 6
+
+    exp  ->  exp '*' . exp   (rule 3)
+
+    NUM         shift, and go to state 1
+
+    exp         go to state 10
+
+state 7
+
+    exp  ->  exp '/' . exp   (rule 4)
+
+    NUM         shift, and go to state 1
+
+    exp         go to state 11
+@end example
+
+As was announced in beginning of the report, @samp{State 8 conflicts:
+1 shift/reduce}:
+
+@example
+state 8
+
+    exp  ->  exp . '+' exp   (rule 1)
+    exp  ->  exp '+' exp .   (rule 1)
+    exp  ->  exp . '-' exp   (rule 2)
+    exp  ->  exp . '*' exp   (rule 3)
+    exp  ->  exp . '/' exp   (rule 4)
+
+    '*'         shift, and go to state 6
+    '/'         shift, and go to state 7
+
+    '/'         [reduce using rule 1 (exp)]
+    $default    reduce using rule 1 (exp)
+@end example
+
+Indeed, there are two actions associated to the lookahead @samp{/}:
+either shifting (and going to state 7), or reducing rule 1.  The
+conflict means that either the grammar is ambiguous, or the parser lacks
+information to make the right decision.  Indeed the grammar is
+ambiguous, as, since we did not specify the precedence of @samp{/}, the
+sentence @samp{NUM + NUM / NUM} can be parsed as @samp{NUM + (NUM /
+NUM)}, which corresponds to shifting @samp{/}, or as @samp{(NUM + NUM) /
+NUM}, which corresponds to reducing rule 1.
+
+Because in @acronym{LALR}(1) parsing a single decision can be made, Bison
+arbitrarily chose to disable the reduction, see @ref{Shift/Reduce, ,
+Shift/Reduce Conflicts}.  Discarded actions are reported in between
+square brackets.
+
+Note that all the previous states had a single possible action: either
+shifting the next token and going to the corresponding state, or
+reducing a single rule.  In the other cases, i.e., when shifting
+@emph{and} reducing is possible or when @emph{several} reductions are
+possible, the lookahead is required to select the action.  State 8 is
+one such state: if the lookahead is @samp{*} or @samp{/} then the action
+is shifting, otherwise the action is reducing rule 1.  In other words,
+the first two items, corresponding to rule 1, are not eligible when the
+lookahead token is @samp{*}, since we specified that @samp{*} has higher
+precedence than @samp{+}.  More generally, some items are eligible only
+with some set of possible lookahead tokens.  When run with
+@option{--report=lookahead}, Bison specifies these lookahead tokens:
+
+@example
+state 8
+
+    exp  ->  exp . '+' exp   (rule 1)
+    exp  ->  exp '+' exp .  [$, '+', '-', '/']   (rule 1)
+    exp  ->  exp . '-' exp   (rule 2)
+    exp  ->  exp . '*' exp   (rule 3)
+    exp  ->  exp . '/' exp   (rule 4)
+
+    '*'         shift, and go to state 6
+    '/'         shift, and go to state 7
+
+    '/'         [reduce using rule 1 (exp)]
+    $default    reduce using rule 1 (exp)
+@end example
+
+The remaining states are similar:
+
+@example
+state 9
+
+    exp  ->  exp . '+' exp   (rule 1)
+    exp  ->  exp . '-' exp   (rule 2)
+    exp  ->  exp '-' exp .   (rule 2)
+    exp  ->  exp . '*' exp   (rule 3)
+    exp  ->  exp . '/' exp   (rule 4)
+
+    '*'         shift, and go to state 6
+    '/'         shift, and go to state 7
+
+    '/'         [reduce using rule 2 (exp)]
+    $default    reduce using rule 2 (exp)
+
+state 10
+
+    exp  ->  exp . '+' exp   (rule 1)
+    exp  ->  exp . '-' exp   (rule 2)
+    exp  ->  exp . '*' exp   (rule 3)
+    exp  ->  exp '*' exp .   (rule 3)
+    exp  ->  exp . '/' exp   (rule 4)
+
+    '/'         shift, and go to state 7
+
+    '/'         [reduce using rule 3 (exp)]
+    $default    reduce using rule 3 (exp)
+
+state 11
+
+    exp  ->  exp . '+' exp   (rule 1)
+    exp  ->  exp . '-' exp   (rule 2)
+    exp  ->  exp . '*' exp   (rule 3)
+    exp  ->  exp . '/' exp   (rule 4)
+    exp  ->  exp '/' exp .   (rule 4)
+
+    '+'         shift, and go to state 4
+    '-'         shift, and go to state 5
+    '*'         shift, and go to state 6
+    '/'         shift, and go to state 7
+
+    '+'         [reduce using rule 4 (exp)]
+    '-'         [reduce using rule 4 (exp)]
+    '*'         [reduce using rule 4 (exp)]
+    '/'         [reduce using rule 4 (exp)]
+    $default    reduce using rule 4 (exp)
+@end example
+
+@noindent
+Observe that state 11 contains conflicts not only due to the lack of
+precedence of @samp{/} with respect to @samp{+}, @samp{-}, and
+@samp{*}, but also because the
+associativity of @samp{/} is not specified.
+
+
+@node Tracing
+@section Tracing Your Parser
+@findex yydebug
+@cindex debugging
+@cindex tracing the parser
+
+If a Bison grammar compiles properly but doesn't do what you want when it
+runs, the @code{yydebug} parser-trace feature can help you figure out why.
+
+There are several means to enable compilation of trace facilities:
+
+@table @asis
+@item the macro @code{YYDEBUG}
+@findex YYDEBUG
+Define the macro @code{YYDEBUG} to a nonzero value when you compile the
+parser.  This is compliant with @acronym{POSIX} Yacc.  You could use
+@samp{-DYYDEBUG=1} as a compiler option or you could put @samp{#define
+YYDEBUG 1} in the prologue of the grammar file (@pxref{Prologue, , The
+Prologue}).
+
+@item the option @option{-t}, @option{--debug}
+Use the @samp{-t} option when you run Bison (@pxref{Invocation,
+,Invoking Bison}).  This is @acronym{POSIX} compliant too.
+
+@item the directive @samp{%debug}
+@findex %debug
+Add the @code{%debug} directive (@pxref{Decl Summary, ,Bison
+Declaration Summary}).  This is a Bison extension, which will prove
+useful when Bison will output parsers for languages that don't use a
+preprocessor.  Unless @acronym{POSIX} and Yacc portability matter to
+you, this is
+the preferred solution.
+@end table
+
+We suggest that you always enable the debug option so that debugging is
+always possible.
+
+The trace facility outputs messages with macro calls of the form
+@code{YYFPRINTF (stderr, @var{format}, @var{args})} where
+@var{format} and @var{args} are the usual @code{printf} format and variadic
+arguments.  If you define @code{YYDEBUG} to a nonzero value but do not
+define @code{YYFPRINTF}, @code{<stdio.h>} is automatically included
+and @code{YYFPRINTF} is defined to @code{fprintf}.
+
+Once you have compiled the program with trace facilities, the way to
+request a trace is to store a nonzero value in the variable @code{yydebug}.
+You can do this by making the C code do it (in @code{main}, perhaps), or
+you can alter the value with a C debugger.
+
+Each step taken by the parser when @code{yydebug} is nonzero produces a
+line or two of trace information, written on @code{stderr}.  The trace
+messages tell you these things:
+
+@itemize @bullet
+@item
+Each time the parser calls @code{yylex}, what kind of token was read.
+
+@item
+Each time a token is shifted, the depth and complete contents of the
+state stack (@pxref{Parser States}).
+
+@item
+Each time a rule is reduced, which rule it is, and the complete contents
+of the state stack afterward.
+@end itemize
+
+To make sense of this information, it helps to refer to the listing file
+produced by the Bison @samp{-v} option (@pxref{Invocation, ,Invoking
+Bison}).  This file shows the meaning of each state in terms of
+positions in various rules, and also what each state will do with each
+possible input token.  As you read the successive trace messages, you
+can see that the parser is functioning according to its specification in
+the listing file.  Eventually you will arrive at the place where
+something undesirable happens, and you will see which parts of the
+grammar are to blame.
+
+The parser file is a C program and you can use C debuggers on it, but it's
+not easy to interpret what it is doing.  The parser function is a
+finite-state machine interpreter, and aside from the actions it executes
+the same code over and over.  Only the values of variables show where in
+the grammar it is working.
+
+@findex YYPRINT
+The debugging information normally gives the token type of each token
+read, but not its semantic value.  You can optionally define a macro
+named @code{YYPRINT} to provide a way to print the value.  If you define
+@code{YYPRINT}, it should take three arguments.  The parser will pass a
+standard I/O stream, the numeric code for the token type, and the token
+value (from @code{yylval}).
+
+Here is an example of @code{YYPRINT} suitable for the multi-function
+calculator (@pxref{Mfcalc Declarations, ,Declarations for @code{mfcalc}}):
+
+@smallexample
+%@{
+  static void print_token_value (FILE *, int, YYSTYPE);
+  #define YYPRINT(file, type, value) print_token_value (file, type, value)
+%@}
+
+@dots{} %% @dots{} %% @dots{}
+
+static void
+print_token_value (FILE *file, int type, YYSTYPE value)
+@{
+  if (type == VAR)
+    fprintf (file, "%s", value.tptr->name);
+  else if (type == NUM)
+    fprintf (file, "%d", value.val);
+@}
+@end smallexample
+
+@c ================================================= Invoking Bison
+
+@node Invocation
+@chapter Invoking Bison
+@cindex invoking Bison
+@cindex Bison invocation
+@cindex options for invoking Bison
+
+The usual way to invoke Bison is as follows:
+
+@example
+bison @var{infile}
+@end example
+
+Here @var{infile} is the grammar file name, which usually ends in
+@samp{.y}.  The parser file's name is made by replacing the @samp{.y}
+with @samp{.tab.c} and removing any leading directory.  Thus, the
+@samp{bison foo.y} file name yields
+@file{foo.tab.c}, and the @samp{bison hack/foo.y} file name yields
+@file{foo.tab.c}.  It's also possible, in case you are writing
+C++ code instead of C in your grammar file, to name it @file{foo.ypp}
+or @file{foo.y++}.  Then, the output files will take an extension like
+the given one as input (respectively @file{foo.tab.cpp} and
+@file{foo.tab.c++}).
+This feature takes effect with all options that manipulate file names like
+@samp{-o} or @samp{-d}.
+
+For example :
+
+@example
+bison -d @var{infile.yxx}
+@end example
+@noindent
+will produce @file{infile.tab.cxx} and @file{infile.tab.hxx}, and
+
+@example
+bison -d -o @var{output.c++} @var{infile.y}
+@end example
+@noindent
+will produce @file{output.c++} and @file{outfile.h++}.
+
+For compatibility with @acronym{POSIX}, the standard Bison
+distribution also contains a shell script called @command{yacc} that
+invokes Bison with the @option{-y} option.
+
+@menu
+* Bison Options::     All the options described in detail,
+                        in alphabetical order by short options.
+* Option Cross Key::  Alphabetical list of long options.
+* Yacc Library::      Yacc-compatible @code{yylex} and @code{main}.
+@end menu
+
+@node Bison Options
+@section Bison Options
+
+Bison supports both traditional single-letter options and mnemonic long
+option names.  Long option names are indicated with @samp{--} instead of
+@samp{-}.  Abbreviations for option names are allowed as long as they
+are unique.  When a long option takes an argument, like
+@samp{--file-prefix}, connect the option name and the argument with
+@samp{=}.
+
+Here is a list of options that can be used with Bison, alphabetized by
+short option.  It is followed by a cross key alphabetized by long
+option.
+
+@c Please, keep this ordered as in `bison --help'.
+@noindent
+Operations modes:
+@table @option
+@item -h
+@itemx --help
+Print a summary of the command-line options to Bison and exit.
+
+@item -V
+@itemx --version
+Print the version number of Bison and exit.
+
+@item --print-localedir
+Print the name of the directory containing locale-dependent data.
+
+@item --print-datadir
+Print the name of the directory containing skeletons and XSLT.
+
+@item -y
+@itemx --yacc
+Act more like the traditional Yacc command.  This can cause
+different diagnostics to be generated, and may change behavior in
+other minor ways.  Most importantly, imitate Yacc's output
+file name conventions, so that the parser output file is called
+@file{y.tab.c}, and the other outputs are called @file{y.output} and
+@file{y.tab.h}.
+Also, if generating an @acronym{LALR}(1) parser in C, generate @code{#define}
+statements in addition to an @code{enum} to associate token numbers with token
+names.
+Thus, the following shell script can substitute for Yacc, and the Bison
+distribution contains such a script for compatibility with @acronym{POSIX}:
+
+@example
+#! /bin/sh
+bison -y "$@@"
+@end example
+
+The @option{-y}/@option{--yacc} option is intended for use with
+traditional Yacc grammars.  If your grammar uses a Bison extension
+like @samp{%glr-parser}, Bison might not be Yacc-compatible even if
+this option is specified.
+
+@item -W
+@itemx --warnings
+Output warnings falling in @var{category}.  @var{category} can be one
+of:
+@table @code
+@item midrule-values
+Warn about mid-rule values that are set but not used within any of the actions
+of the parent rule.
+For example, warn about unused @code{$2} in:
+
+@example
+exp: '1' @{ $$ = 1; @} '+' exp @{ $$ = $1 + $4; @};
+@end example
+
+Also warn about mid-rule values that are used but not set.
+For example, warn about unset @code{$$} in the mid-rule action in:
+
+@example
+ exp: '1' @{ $1 = 1; @} '+' exp @{ $$ = $2 + $4; @};
+@end example
+
+These warnings are not enabled by default since they sometimes prove to
+be false alarms in existing grammars employing the Yacc constructs
+@code{$0} or @code{$-@var{n}} (where @var{n} is some positive integer).
+
+
+@item yacc
+Incompatibilities with @acronym{POSIX} Yacc.
+
+@item all
+All the warnings.
+@item none
+Turn off all the warnings.
+@item error
+Treat warnings as errors.
+@end table
+
+A category can be turned off by prefixing its name with @samp{no-}.  For
+instance, @option{-Wno-syntax} will hide the warnings about unused
+variables.
+@end table
+
+@noindent
+Tuning the parser:
+
+@table @option
+@item -t
+@itemx --debug
+In the parser file, define the macro @code{YYDEBUG} to 1 if it is not
+already defined, so that the debugging facilities are compiled.
+@xref{Tracing, ,Tracing Your Parser}.
+
+@item -L @var{language}
+@itemx --language=@var{language}
+Specify the programming language for the generated parser, as if
+@code{%language} was specified (@pxref{Decl Summary, , Bison Declaration
+Summary}).  Currently supported languages include C, C++, and Java.
+@var{language} is case-insensitive.
+
+This option is experimental and its effect may be modified in future
+releases.
+
+@item --locations
+Pretend that @code{%locations} was specified.  @xref{Decl Summary}.
+
+@item -p @var{prefix}
+@itemx --name-prefix=@var{prefix}
+Pretend that @code{%name-prefix "@var{prefix}"} was specified.
+@xref{Decl Summary}.
+
+@item -l
+@itemx --no-lines
+Don't put any @code{#line} preprocessor commands in the parser file.
+Ordinarily Bison puts them in the parser file so that the C compiler
+and debuggers will associate errors with your source file, the
+grammar file.  This option causes them to associate errors with the
+parser file, treating it as an independent source file in its own right.
+
+@item -S @var{file}
+@itemx --skeleton=@var{file}
+Specify the skeleton to use, similar to @code{%skeleton}
+(@pxref{Decl Summary, , Bison Declaration Summary}).
+
+@c You probably don't need this option unless you are developing Bison.
+@c You should use @option{--language} if you want to specify the skeleton for a
+@c different language, because it is clearer and because it will always
+@c choose the correct skeleton for non-deterministic or push parsers.
+
+If @var{file} does not contain a @code{/}, @var{file} is the name of a skeleton
+file in the Bison installation directory.
+If it does, @var{file} is an absolute file name or a file name relative to the
+current working directory.
+This is similar to how most shells resolve commands.
+
+@item -k
+@itemx --token-table
+Pretend that @code{%token-table} was specified.  @xref{Decl Summary}.
+@end table
+
+@noindent
+Adjust the output:
+
+@table @option
+@item --defines[=@var{file}]
+Pretend that @code{%defines} was specified, i.e., write an extra output
+file containing macro definitions for the token type names defined in
+the grammar, as well as a few other declarations.  @xref{Decl Summary}.
+
+@item -d
+This is the same as @code{--defines} except @code{-d} does not accept a
+@var{file} argument since POSIX Yacc requires that @code{-d} can be bundled
+with other short options.
+
+@item -b @var{file-prefix}
+@itemx --file-prefix=@var{prefix}
+Pretend that @code{%file-prefix} was specified, i.e., specify prefix to use
+for all Bison output file names.  @xref{Decl Summary}.
+
+@item -r @var{things}
+@itemx --report=@var{things}
+Write an extra output file containing verbose description of the comma
+separated list of @var{things} among:
+
+@table @code
+@item state
+Description of the grammar, conflicts (resolved and unresolved), and
+@acronym{LALR} automaton.
+
+@item lookahead
+Implies @code{state} and augments the description of the automaton with
+each rule's lookahead set.
+
+@item itemset
+Implies @code{state} and augments the description of the automaton with
+the full set of items for each state, instead of its core only.
+@end table
+
+@item --report-file=@var{file}
+Specify the @var{file} for the verbose description.
+
+@item -v
+@itemx --verbose
+Pretend that @code{%verbose} was specified, i.e., write an extra output
+file containing verbose descriptions of the grammar and
+parser.  @xref{Decl Summary}.
+
+@item -o @var{file}
+@itemx --output=@var{file}
+Specify the @var{file} for the parser file.
+
+The other output files' names are constructed from @var{file} as
+described under the @samp{-v} and @samp{-d} options.
+
+@item -g[@var{file}]
+@itemx --graph[=@var{file}]
+Output a graphical representation of the @acronym{LALR}(1) grammar
+automaton computed by Bison, in @uref{http://www.graphviz.org/, Graphviz}
+@uref{http://www.graphviz.org/doc/info/lang.html, @acronym{DOT}} format.
+@code{@var{file}} is optional.
+If omitted and the grammar file is @file{foo.y}, the output file will be
+@file{foo.dot}.
+
+@item -x[@var{file}]
+@itemx --xml[=@var{file}]
+Output an XML report of the @acronym{LALR}(1) automaton computed by Bison.
+@code{@var{file}} is optional.
+If omitted and the grammar file is @file{foo.y}, the output file will be
+@file{foo.xml}.
+(The current XML schema is experimental and may evolve.
+More user feedback will help to stabilize it.)
+@end table
+
+@node Option Cross Key
+@section Option Cross Key
+
+@c FIXME: How about putting the directives too?
+Here is a list of options, alphabetized by long option, to help you find
+the corresponding short option.
+
+@multitable {@option{--defines=@var{defines-file}}} {@option{-b @var{file-prefix}XXX}}
+@headitem Long Option @tab Short Option
+@include cross-options.texi
+@end multitable
+
+@node Yacc Library
+@section Yacc Library
+
+The Yacc library contains default implementations of the
+@code{yyerror} and @code{main} functions.  These default
+implementations are normally not useful, but @acronym{POSIX} requires
+them.  To use the Yacc library, link your program with the
+@option{-ly} option.  Note that Bison's implementation of the Yacc
+library is distributed under the terms of the @acronym{GNU} General
+Public License (@pxref{Copying}).
+
+If you use the Yacc library's @code{yyerror} function, you should
+declare @code{yyerror} as follows:
+
+@example
+int yyerror (char const *);
+@end example
+
+Bison ignores the @code{int} value returned by this @code{yyerror}.
+If you use the Yacc library's @code{main} function, your
+@code{yyparse} function should have the following type signature:
+
+@example
+int yyparse (void);
+@end example
+
+@c ================================================= C++ Bison
+
+@node Other Languages
+@chapter Parsers Written In Other Languages
+
+@menu
+* C++ Parsers::                 The interface to generate C++ parser classes
+* Java Parsers::                The interface to generate Java parser classes
+@end menu
+
+@node C++ Parsers
+@section C++ Parsers
+
+@menu
+* C++ Bison Interface::         Asking for C++ parser generation
+* C++ Semantic Values::         %union vs. C++
+* C++ Location Values::         The position and location classes
+* C++ Parser Interface::        Instantiating and running the parser
+* C++ Scanner Interface::       Exchanges between yylex and parse
+* A Complete C++ Example::      Demonstrating their use
+@end menu
+
+@node C++ Bison Interface
+@subsection C++ Bison Interface
+@c - %skeleton "lalr1.cc"
+@c - Always pure
+@c - initial action
+
+The C++ @acronym{LALR}(1) parser is selected using the skeleton directive,
+@samp{%skeleton "lalr1.c"}, or the synonymous command-line option
+@option{--skeleton=lalr1.c}.
+@xref{Decl Summary}.
+
+When run, @command{bison} will create several entities in the @samp{yy}
+namespace.
+@findex %define namespace
+Use the @samp{%define namespace} directive to change the namespace name, see
+@ref{Decl Summary}.
+The various classes are generated in the following files:
+
+@table @file
+@item position.hh
+@itemx location.hh
+The definition of the classes @code{position} and @code{location},
+used for location tracking.  @xref{C++ Location Values}.
+
+@item stack.hh
+An auxiliary class @code{stack} used by the parser.
+
+@item @var{file}.hh
+@itemx @var{file}.cc
+(Assuming the extension of the input file was @samp{.yy}.)  The
+declaration and implementation of the C++ parser class.  The basename
+and extension of these two files follow the same rules as with regular C
+parsers (@pxref{Invocation}).
+
+The header is @emph{mandatory}; you must either pass
+@option{-d}/@option{--defines} to @command{bison}, or use the
+@samp{%defines} directive.
+@end table
+
+All these files are documented using Doxygen; run @command{doxygen}
+for a complete and accurate documentation.
+
+@node C++ Semantic Values
+@subsection C++ Semantic Values
+@c - No objects in unions
+@c - YYSTYPE
+@c - Printer and destructor
+
+The @code{%union} directive works as for C, see @ref{Union Decl, ,The
+Collection of Value Types}.  In particular it produces a genuine
+@code{union}@footnote{In the future techniques to allow complex types
+within pseudo-unions (similar to Boost variants) might be implemented to
+alleviate these issues.}, which have a few specific features in C++.
+@itemize @minus
+@item
+The type @code{YYSTYPE} is defined but its use is discouraged: rather
+you should refer to the parser's encapsulated type
+@code{yy::parser::semantic_type}.
+@item
+Non POD (Plain Old Data) types cannot be used.  C++ forbids any
+instance of classes with constructors in unions: only @emph{pointers}
+to such objects are allowed.
+@end itemize
+
+Because objects have to be stored via pointers, memory is not
+reclaimed automatically: using the @code{%destructor} directive is the
+only means to avoid leaks.  @xref{Destructor Decl, , Freeing Discarded
+Symbols}.
+
+
+@node C++ Location Values
+@subsection C++ Location Values
+@c - %locations
+@c - class Position
+@c - class Location
+@c - %define filename_type "const symbol::Symbol"
+
+When the directive @code{%locations} is used, the C++ parser supports
+location tracking, see @ref{Locations, , Locations Overview}.  Two
+auxiliary classes define a @code{position}, a single point in a file,
+and a @code{location}, a range composed of a pair of
+@code{position}s (possibly spanning several files).
+
+@deftypemethod {position} {std::string*} file
+The name of the file.  It will always be handled as a pointer, the
+parser will never duplicate nor deallocate it.  As an experimental
+feature you may change it to @samp{@var{type}*} using @samp{%define
+filename_type "@var{type}"}.
+@end deftypemethod
+
+@deftypemethod {position} {unsigned int} line
+The line, starting at 1.
+@end deftypemethod
+
+@deftypemethod {position} {unsigned int} lines (int @var{height} = 1)
+Advance by @var{height} lines, resetting the column number.
+@end deftypemethod
+
+@deftypemethod {position} {unsigned int} column
+The column, starting at 0.
+@end deftypemethod
+
+@deftypemethod {position} {unsigned int} columns (int @var{width} = 1)
+Advance by @var{width} columns, without changing the line number.
+@end deftypemethod
+
+@deftypemethod {position} {position&} operator+= (position& @var{pos}, int @var{width})
+@deftypemethodx {position} {position} operator+ (const position& @var{pos}, int @var{width})
+@deftypemethodx {position} {position&} operator-= (const position& @var{pos}, int @var{width})
+@deftypemethodx {position} {position} operator- (position& @var{pos}, int @var{width})
+Various forms of syntactic sugar for @code{columns}.
+@end deftypemethod
+
+@deftypemethod {position} {position} operator<< (std::ostream @var{o}, const position& @var{p})
+Report @var{p} on @var{o} like this:
+@samp{@var{file}:@var{line}.@var{column}}, or
+@samp{@var{line}.@var{column}} if @var{file} is null.
+@end deftypemethod
+
+@deftypemethod {location} {position} begin
+@deftypemethodx {location} {position} end
+The first, inclusive, position of the range, and the first beyond.
+@end deftypemethod
+
+@deftypemethod {location} {unsigned int} columns (int @var{width} = 1)
+@deftypemethodx {location} {unsigned int} lines (int @var{height} = 1)
+Advance the @code{end} position.
+@end deftypemethod
+
+@deftypemethod {location} {location} operator+ (const location& @var{begin}, const location& @var{end})
+@deftypemethodx {location} {location} operator+ (const location& @var{begin}, int @var{width})
+@deftypemethodx {location} {location} operator+= (const location& @var{loc}, int @var{width})
+Various forms of syntactic sugar.
+@end deftypemethod
+
+@deftypemethod {location} {void} step ()
+Move @code{begin} onto @code{end}.
+@end deftypemethod
+
+
+@node C++ Parser Interface
+@subsection C++ Parser Interface
+@c - define parser_class_name
+@c - Ctor
+@c - parse, error, set_debug_level, debug_level, set_debug_stream,
+@c   debug_stream.
+@c - Reporting errors
+
+The output files @file{@var{output}.hh} and @file{@var{output}.cc}
+declare and define the parser class in the namespace @code{yy}.  The
+class name defaults to @code{parser}, but may be changed using
+@samp{%define parser_class_name "@var{name}"}.  The interface of
+this class is detailed below.  It can be extended using the
+@code{%parse-param} feature: its semantics is slightly changed since
+it describes an additional member of the parser class, and an
+additional argument for its constructor.
+
+@defcv {Type} {parser} {semantic_value_type}
+@defcvx {Type} {parser} {location_value_type}
+The types for semantics value and locations.
+@end defcv
+
+@deftypemethod {parser} {} parser (@var{type1} @var{arg1}, ...)
+Build a new parser object.  There are no arguments by default, unless
+@samp{%parse-param @{@var{type1} @var{arg1}@}} was used.
+@end deftypemethod
+
+@deftypemethod {parser} {int} parse ()
+Run the syntactic analysis, and return 0 on success, 1 otherwise.
+@end deftypemethod
+
+@deftypemethod {parser} {std::ostream&} debug_stream ()
+@deftypemethodx {parser} {void} set_debug_stream (std::ostream& @var{o})
+Get or set the stream used for tracing the parsing.  It defaults to
+@code{std::cerr}.
+@end deftypemethod
+
+@deftypemethod {parser} {debug_level_type} debug_level ()
+@deftypemethodx {parser} {void} set_debug_level (debug_level @var{l})
+Get or set the tracing level.  Currently its value is either 0, no trace,
+or nonzero, full tracing.
+@end deftypemethod
+
+@deftypemethod {parser} {void} error (const location_type& @var{l}, const std::string& @var{m})
+The definition for this member function must be supplied by the user:
+the parser uses it to report a parser error occurring at @var{l},
+described by @var{m}.
+@end deftypemethod
+
+
+@node C++ Scanner Interface
+@subsection C++ Scanner Interface
+@c - prefix for yylex.
+@c - Pure interface to yylex
+@c - %lex-param
+
+The parser invokes the scanner by calling @code{yylex}.  Contrary to C
+parsers, C++ parsers are always pure: there is no point in using the
+@code{%define api.pure} directive.  Therefore the interface is as follows.
+
+@deftypemethod {parser} {int} yylex (semantic_value_type& @var{yylval}, location_type& @var{yylloc}, @var{type1} @var{arg1}, ...)
+Return the next token.  Its type is the return value, its semantic
+value and location being @var{yylval} and @var{yylloc}.  Invocations of
+@samp{%lex-param @{@var{type1} @var{arg1}@}} yield additional arguments.
+@end deftypemethod
+
+
+@node A Complete C++ Example
+@subsection A Complete C++ Example
+
+This section demonstrates the use of a C++ parser with a simple but
+complete example.  This example should be available on your system,
+ready to compile, in the directory @dfn{../bison/examples/calc++}.  It
+focuses on the use of Bison, therefore the design of the various C++
+classes is very naive: no accessors, no encapsulation of members etc.
+We will use a Lex scanner, and more precisely, a Flex scanner, to
+demonstrate the various interaction.  A hand written scanner is
+actually easier to interface with.
+
+@menu
+* Calc++ --- C++ Calculator::   The specifications
+* Calc++ Parsing Driver::       An active parsing context
+* Calc++ Parser::               A parser class
+* Calc++ Scanner::              A pure C++ Flex scanner
+* Calc++ Top Level::            Conducting the band
+@end menu
+
+@node Calc++ --- C++ Calculator
+@subsubsection Calc++ --- C++ Calculator
+
+Of course the grammar is dedicated to arithmetics, a single
+expression, possibly preceded by variable assignments.  An
+environment containing possibly predefined variables such as
+@code{one} and @code{two}, is exchanged with the parser.  An example
+of valid input follows.
+
+@example
+three := 3
+seven := one + two * three
+seven * seven
+@end example
+
+@node Calc++ Parsing Driver
+@subsubsection Calc++ Parsing Driver
+@c - An env
+@c - A place to store error messages
+@c - A place for the result
+
+To support a pure interface with the parser (and the scanner) the
+technique of the ``parsing context'' is convenient: a structure
+containing all the data to exchange.  Since, in addition to simply
+launch the parsing, there are several auxiliary tasks to execute (open
+the file for parsing, instantiate the parser etc.), we recommend
+transforming the simple parsing context structure into a fully blown
+@dfn{parsing driver} class.
+
+The declaration of this driver class, @file{calc++-driver.hh}, is as
+follows.  The first part includes the CPP guard and imports the
+required standard library components, and the declaration of the parser
+class.
+
+@comment file: calc++-driver.hh
+@example
+#ifndef CALCXX_DRIVER_HH
+# define CALCXX_DRIVER_HH
+# include <string>
+# include <map>
+# include "calc++-parser.hh"
+@end example
+
+
+@noindent
+Then comes the declaration of the scanning function.  Flex expects
+the signature of @code{yylex} to be defined in the macro
+@code{YY_DECL}, and the C++ parser expects it to be declared.  We can
+factor both as follows.
+
+@comment file: calc++-driver.hh
+@example
+// Tell Flex the lexer's prototype ...
+# define YY_DECL                                        \
+  yy::calcxx_parser::token_type                         \
+  yylex (yy::calcxx_parser::semantic_type* yylval,      \
+         yy::calcxx_parser::location_type* yylloc,      \
+         calcxx_driver& driver)
+// ... and declare it for the parser's sake.
+YY_DECL;
+@end example
+
+@noindent
+The @code{calcxx_driver} class is then declared with its most obvious
+members.
+
+@comment file: calc++-driver.hh
+@example
+// Conducting the whole scanning and parsing of Calc++.
+class calcxx_driver
+@{
+public:
+  calcxx_driver ();
+  virtual ~calcxx_driver ();
+
+  std::map<std::string, int> variables;
+
+  int result;
+@end example
+
+@noindent
+To encapsulate the coordination with the Flex scanner, it is useful to
+have two members function to open and close the scanning phase.
+
+@comment file: calc++-driver.hh
+@example
+  // Handling the scanner.
+  void scan_begin ();
+  void scan_end ();
+  bool trace_scanning;
+@end example
+
+@noindent
+Similarly for the parser itself.
+
+@comment file: calc++-driver.hh
+@example
+  // Run the parser.  Return 0 on success.
+  int parse (const std::string& f);
+  std::string file;
+  bool trace_parsing;
+@end example
+
+@noindent
+To demonstrate pure handling of parse errors, instead of simply
+dumping them on the standard error output, we will pass them to the
+compiler driver using the following two member functions.  Finally, we
+close the class declaration and CPP guard.
+
+@comment file: calc++-driver.hh
+@example
+  // Error handling.
+  void error (const yy::location& l, const std::string& m);
+  void error (const std::string& m);
+@};
+#endif // ! CALCXX_DRIVER_HH
+@end example
+
+The implementation of the driver is straightforward.  The @code{parse}
+member function deserves some attention.  The @code{error} functions
+are simple stubs, they should actually register the located error
+messages and set error state.
+
+@comment file: calc++-driver.cc
+@example
+#include "calc++-driver.hh"
+#include "calc++-parser.hh"
+
+calcxx_driver::calcxx_driver ()
+  : trace_scanning (false), trace_parsing (false)
+@{
+  variables["one"] = 1;
+  variables["two"] = 2;
+@}
+
+calcxx_driver::~calcxx_driver ()
+@{
+@}
+
+int
+calcxx_driver::parse (const std::string &f)
+@{
+  file = f;
+  scan_begin ();
+  yy::calcxx_parser parser (*this);
+  parser.set_debug_level (trace_parsing);
+  int res = parser.parse ();
+  scan_end ();
+  return res;
+@}
+
+void
+calcxx_driver::error (const yy::location& l, const std::string& m)
+@{
+  std::cerr << l << ": " << m << std::endl;
+@}
+
+void
+calcxx_driver::error (const std::string& m)
+@{
+  std::cerr << m << std::endl;
+@}
+@end example
+
+@node Calc++ Parser
+@subsubsection Calc++ Parser
+
+The parser definition file @file{calc++-parser.yy} starts by asking for
+the C++ LALR(1) skeleton, the creation of the parser header file, and
+specifies the name of the parser class.  Because the C++ skeleton
+changed several times, it is safer to require the version you designed
+the grammar for.
+
+@comment file: calc++-parser.yy
+@example
+%skeleton "lalr1.cc"                          /*  -*- C++ -*- */
+%require "@value{VERSION}"
+%defines
+%define parser_class_name "calcxx_parser"
+@end example
+
+@noindent
+@findex %code requires
+Then come the declarations/inclusions needed to define the
+@code{%union}.  Because the parser uses the parsing driver and
+reciprocally, both cannot include the header of the other.  Because the
+driver's header needs detailed knowledge about the parser class (in
+particular its inner types), it is the parser's header which will simply
+use a forward declaration of the driver.
+@xref{Decl Summary, ,%code}.
+
+@comment file: calc++-parser.yy
+@example
+%code requires @{
+# include <string>
+class calcxx_driver;
+@}
+@end example
+
+@noindent
+The driver is passed by reference to the parser and to the scanner.
+This provides a simple but effective pure interface, not relying on
+global variables.
+
+@comment file: calc++-parser.yy
+@example
+// The parsing context.
+%parse-param @{ calcxx_driver& driver @}
+%lex-param   @{ calcxx_driver& driver @}
+@end example
+
+@noindent
+Then we request the location tracking feature, and initialize the
+first location's file name.  Afterwards new locations are computed
+relatively to the previous locations: the file name will be
+automatically propagated.
+
+@comment file: calc++-parser.yy
+@example
+%locations
+%initial-action
+@{
+  // Initialize the initial location.
+  @@$.begin.filename = @@$.end.filename = &driver.file;
+@};
+@end example
+
+@noindent
+Use the two following directives to enable parser tracing and verbose
+error messages.
+
+@comment file: calc++-parser.yy
+@example
+%debug
+%error-verbose
+@end example
+
+@noindent
+Semantic values cannot use ``real'' objects, but only pointers to
+them.
+
+@comment file: calc++-parser.yy
+@example
+// Symbols.
+%union
+@{
+  int          ival;
+  std::string *sval;
+@};
+@end example
+
+@noindent
+@findex %code
+The code between @samp{%code @{} and @samp{@}} is output in the
+@file{*.cc} file; it needs detailed knowledge about the driver.
+
+@comment file: calc++-parser.yy
+@example
+%code @{
+# include "calc++-driver.hh"
+@}
+@end example
+
+
+@noindent
+The token numbered as 0 corresponds to end of file; the following line
+allows for nicer error messages referring to ``end of file'' instead
+of ``$end''.  Similarly user friendly named are provided for each
+symbol.  Note that the tokens names are prefixed by @code{TOKEN_} to
+avoid name clashes.
+
+@comment file: calc++-parser.yy
+@example
+%token        END      0 "end of file"
+%token        ASSIGN     ":="
+%token <sval> IDENTIFIER "identifier"
+%token <ival> NUMBER     "number"
+%type  <ival> exp
+@end example
+
+@noindent
+To enable memory deallocation during error recovery, use
+@code{%destructor}.
+
+@c FIXME: Document %printer, and mention that it takes a braced-code operand.
+@comment file: calc++-parser.yy
+@example
+%printer    @{ debug_stream () << *$$; @} "identifier"
+%destructor @{ delete $$; @} "identifier"
+
+%printer    @{ debug_stream () << $$; @} <ival>
+@end example
+
+@noindent
+The grammar itself is straightforward.
+
+@comment file: calc++-parser.yy
+@example
+%%
+%start unit;
+unit: assignments exp  @{ driver.result = $2; @};
+
+assignments: assignments assignment @{@}
+           | /* Nothing.  */        @{@};
+
+assignment:
+     "identifier" ":=" exp
+       @{ driver.variables[*$1] = $3; delete $1; @};
+
+%left '+' '-';
+%left '*' '/';
+exp: exp '+' exp   @{ $$ = $1 + $3; @}
+   | exp '-' exp   @{ $$ = $1 - $3; @}
+   | exp '*' exp   @{ $$ = $1 * $3; @}
+   | exp '/' exp   @{ $$ = $1 / $3; @}
+   | "identifier"  @{ $$ = driver.variables[*$1]; delete $1; @}
+   | "number"      @{ $$ = $1; @};
+%%
+@end example
+
+@noindent
+Finally the @code{error} member function registers the errors to the
+driver.
+
+@comment file: calc++-parser.yy
+@example
+void
+yy::calcxx_parser::error (const yy::calcxx_parser::location_type& l,
+                          const std::string& m)
+@{
+  driver.error (l, m);
+@}
+@end example
+
+@node Calc++ Scanner
+@subsubsection Calc++ Scanner
+
+The Flex scanner first includes the driver declaration, then the
+parser's to get the set of defined tokens.
+
+@comment file: calc++-scanner.ll
+@example
+%@{                                            /* -*- C++ -*- */
+# include <cstdlib>
+# include <errno.h>
+# include <limits.h>
+# include <string>
+# include "calc++-driver.hh"
+# include "calc++-parser.hh"
+
+/* Work around an incompatibility in flex (at least versions
+   2.5.31 through 2.5.33): it generates code that does
+   not conform to C89.  See Debian bug 333231
+   <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=333231>.  */
+# undef yywrap
+# define yywrap() 1
+
+/* By default yylex returns int, we use token_type.
+   Unfortunately yyterminate by default returns 0, which is
+   not of token_type.  */
+#define yyterminate() return token::END
+%@}
+@end example
+
+@noindent
+Because there is no @code{#include}-like feature we don't need
+@code{yywrap}, we don't need @code{unput} either, and we parse an
+actual file, this is not an interactive session with the user.
+Finally we enable the scanner tracing features.
+
+@comment file: calc++-scanner.ll
+@example
+%option noyywrap nounput batch debug
+@end example
+
+@noindent
+Abbreviations allow for more readable rules.
+
+@comment file: calc++-scanner.ll
+@example
+id    [a-zA-Z][a-zA-Z_0-9]*
+int   [0-9]+
+blank [ \t]
+@end example
+
+@noindent
+The following paragraph suffices to track locations accurately.  Each
+time @code{yylex} is invoked, the begin position is moved onto the end
+position.  Then when a pattern is matched, the end position is
+advanced of its width.  In case it matched ends of lines, the end
+cursor is adjusted, and each time blanks are matched, the begin cursor
+is moved onto the end cursor to effectively ignore the blanks
+preceding tokens.  Comments would be treated equally.
+
+@comment file: calc++-scanner.ll
+@example
+%@{
+# define YY_USER_ACTION  yylloc->columns (yyleng);
+%@}
+%%
+%@{
+  yylloc->step ();
+%@}
+@{blank@}+   yylloc->step ();
+[\n]+      yylloc->lines (yyleng); yylloc->step ();
+@end example
+
+@noindent
+The rules are simple, just note the use of the driver to report errors.
+It is convenient to use a typedef to shorten
+@code{yy::calcxx_parser::token::identifier} into
+@code{token::identifier} for instance.
+
+@comment file: calc++-scanner.ll
+@example
+%@{
+  typedef yy::calcxx_parser::token token;
+%@}
+           /* Convert ints to the actual type of tokens.  */
+[-+*/]     return yy::calcxx_parser::token_type (yytext[0]);
+":="       return token::ASSIGN;
+@{int@}      @{
+  errno = 0;
+  long n = strtol (yytext, NULL, 10);
+  if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
+    driver.error (*yylloc, "integer is out of range");
+  yylval->ival = n;
+  return token::NUMBER;
+@}
+@{id@}       yylval->sval = new std::string (yytext); return token::IDENTIFIER;
+.          driver.error (*yylloc, "invalid character");
+%%
+@end example
+
+@noindent
+Finally, because the scanner related driver's member function depend
+on the scanner's data, it is simpler to implement them in this file.
+
+@comment file: calc++-scanner.ll
+@example
+void
+calcxx_driver::scan_begin ()
+@{
+  yy_flex_debug = trace_scanning;
+  if (file == "-")
+    yyin = stdin;
+  else if (!(yyin = fopen (file.c_str (), "r")))
+    @{
+      error (std::string ("cannot open ") + file);
+      exit (1);
+    @}
+@}
+
+void
+calcxx_driver::scan_end ()
+@{
+  fclose (yyin);
+@}
+@end example
+
+@node Calc++ Top Level
+@subsubsection Calc++ Top Level
+
+The top level file, @file{calc++.cc}, poses no problem.
+
+@comment file: calc++.cc
+@example
+#include <iostream>
+#include "calc++-driver.hh"
+
+int
+main (int argc, char *argv[])
+@{
+  calcxx_driver driver;
+  for (++argv; argv[0]; ++argv)
+    if (*argv == std::string ("-p"))
+      driver.trace_parsing = true;
+    else if (*argv == std::string ("-s"))
+      driver.trace_scanning = true;
+    else if (!driver.parse (*argv))
+      std::cout << driver.result << std::endl;
+@}
+@end example
+
+@node Java Parsers
+@section Java Parsers
+
+@menu
+* Java Bison Interface::        Asking for Java parser generation
+* Java Semantic Values::        %type and %token vs. Java
+* Java Location Values::        The position and location classes
+* Java Parser Interface::       Instantiating and running the parser
+* Java Scanner Interface::      Specifying the scanner for the parser
+* Java Action Features::        Special features for use in actions
+* Java Differences::            Differences between C/C++ and Java Grammars
+* Java Declarations Summary::   List of Bison declarations used with Java
+@end menu
+
+@node Java Bison Interface
+@subsection Java Bison Interface
+@c - %language "Java"
+
+(The current Java interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+
+The Java parser skeletons are selected using the @code{%language "Java"}
+directive or the @option{-L java}/@option{--language=java} option.
+
+@c FIXME: Documented bug.
+When generating a Java parser, @code{bison @var{basename}.y} will create
+a single Java source file named @file{@var{basename}.java}.  Using an
+input file without a @file{.y} suffix is currently broken.  The basename
+of the output file can be changed by the @code{%file-prefix} directive
+or the @option{-p}/@option{--name-prefix} option.  The entire output file
+name can be changed by the @code{%output} directive or the
+@option{-o}/@option{--output} option.  The output file contains a single
+class for the parser.
+
+You can create documentation for generated parsers using Javadoc.
+
+Contrary to C parsers, Java parsers do not use global variables; the
+state of the parser is always local to an instance of the parser class.
+Therefore, all Java parsers are ``pure'', and the @code{%pure-parser}
+and @code{%define api.pure} directives does not do anything when used in
+Java.
+
+Push parsers are currently unsupported in Java and @code{%define
+api.push_pull} have no effect.
+
+@acronym{GLR} parsers are currently unsupported in Java.  Do not use the
+@code{glr-parser} directive.
+
+No header file can be generated for Java parsers.  Do not use the
+@code{%defines} directive or the @option{-d}/@option{--defines} options.
+
+@c FIXME: Possible code change.
+Currently, support for debugging and verbose errors are always compiled
+in.  Thus the @code{%debug} and @code{%token-table} directives and the
+@option{-t}/@option{--debug} and @option{-k}/@option{--token-table}
+options have no effect.  This may change in the future to eliminate
+unused code in the generated parser, so use @code{%debug} and
+@code{%verbose-error} explicitly if needed.  Also, in the future the
+@code{%token-table} directive might enable a public interface to
+access the token names and codes.
+
+@node Java Semantic Values
+@subsection Java Semantic Values
+@c - No %union, specify type in %type/%token.
+@c - YYSTYPE
+@c - Printer and destructor
+
+There is no @code{%union} directive in Java parsers.  Instead, the
+semantic values' types (class names) should be specified in the
+@code{%type} or @code{%token} directive:
+
+@example
+%type <Expression> expr assignment_expr term factor
+%type <Integer> number
+@end example
+
+By default, the semantic stack is declared to have @code{Object} members,
+which means that the class types you specify can be of any class.
+To improve the type safety of the parser, you can declare the common
+superclass of all the semantic values using the @code{%define stype}
+directive.  For example, after the following declaration:
+
+@example
+%define stype "ASTNode"
+@end example
+
+@noindent
+any @code{%type} or @code{%token} specifying a semantic type which
+is not a subclass of ASTNode, will cause a compile-time error.
+
+@c FIXME: Documented bug.
+Types used in the directives may be qualified with a package name.
+Primitive data types are accepted for Java version 1.5 or later.  Note
+that in this case the autoboxing feature of Java 1.5 will be used.
+Generic types may not be used; this is due to a limitation in the
+implementation of Bison, and may change in future releases.
+
+Java parsers do not support @code{%destructor}, since the language
+adopts garbage collection.  The parser will try to hold references
+to semantic values for as little time as needed.
+
+Java parsers do not support @code{%printer}, as @code{toString()}
+can be used to print the semantic values.  This however may change
+(in a backwards-compatible way) in future versions of Bison.
+
+
+@node Java Location Values
+@subsection Java Location Values
+@c - %locations
+@c - class Position
+@c - class Location
+
+When the directive @code{%locations} is used, the Java parser
+supports location tracking, see @ref{Locations, , Locations Overview}.
+An auxiliary user-defined class defines a @dfn{position}, a single point
+in a file; Bison itself defines a class representing a @dfn{location},
+a range composed of a pair of positions (possibly spanning several
+files).  The location class is an inner class of the parser; the name
+is @code{Location} by default, and may also be renamed using
+@code{%define location_type "@var{class-name}}.
+
+The location class treats the position as a completely opaque value.
+By default, the class name is @code{Position}, but this can be changed
+with @code{%define position_type "@var{class-name}"}.  This class must
+be supplied by the user.
+
+
+@deftypeivar {Location} {Position} begin
+@deftypeivarx {Location} {Position} end
+The first, inclusive, position of the range, and the first beyond.
+@end deftypeivar
+
+@deftypeop {Constructor} {Location} {} Location (Position @var{loc})
+Create a @code{Location} denoting an empty range located at a given point. 
+@end deftypeop
+
+@deftypeop {Constructor} {Location} {} Location (Position @var{begin}, Position @var{end})
+Create a @code{Location} from the endpoints of the range.
+@end deftypeop
+
+@deftypemethod {Location} {String} toString ()
+Prints the range represented by the location.  For this to work
+properly, the position class should override the @code{equals} and
+@code{toString} methods appropriately.
+@end deftypemethod
+
+
+@node Java Parser Interface
+@subsection Java Parser Interface
+@c - define parser_class_name
+@c - Ctor
+@c - parse, error, set_debug_level, debug_level, set_debug_stream,
+@c   debug_stream.
+@c - Reporting errors
+
+The name of the generated parser class defaults to @code{YYParser}.  The
+@code{YY} prefix may be changed using the @code{%name-prefix} directive
+or the @option{-p}/@option{--name-prefix} option.  Alternatively, use
+@code{%define parser_class_name "@var{name}"} to give a custom name to
+the class.  The interface of this class is detailed below.
+
+By default, the parser class has package visibility.  A declaration
+@code{%define public} will change to public visibility.  Remember that,
+according to the Java language specification, the name of the @file{.java}
+file should match the name of the class in this case.  Similarly, you can
+use @code{abstract}, @code{final} and @code{strictfp} with the
+@code{%define} declaration to add other modifiers to the parser class.
+
+The Java package name of the parser class can be specified using the
+@code{%define package} directive.  The superclass and the implemented
+interfaces of the parser class can be specified with the @code{%define
+extends} and @code{%define implements} directives.
+
+The parser class defines an inner class, @code{Location}, that is used
+for location tracking (see @ref{Java Location Values}), and a inner
+interface, @code{Lexer} (see @ref{Java Scanner Interface}).  Other than
+these inner class/interface, and the members described in the interface
+below, all the other members and fields are preceded with a @code{yy} or
+@code{YY} prefix to avoid clashes with user code.
+
+@c FIXME: The following constants and variables are still undocumented:
+@c @code{bisonVersion}, @code{bisonSkeleton} and @code{errorVerbose}.
+
+The parser class can be extended using the @code{%parse-param}
+directive. Each occurrence of the directive will add a @code{protected
+final} field to the parser class, and an argument to its constructor,
+which initialize them automatically.
+
+Token names defined by @code{%token} and the predefined @code{EOF} token
+name are added as constant fields to the parser class.
+
+@deftypeop {Constructor} {YYParser} {} YYParser (@var{lex_param}, @dots{}, @var{parse_param}, @dots{})
+Build a new parser object with embedded @code{%code lexer}.  There are
+no parameters, unless @code{%parse-param}s and/or @code{%lex-param}s are
+used.
+@end deftypeop
+
+@deftypeop {Constructor} {YYParser} {} YYParser (Lexer @var{lexer}, @var{parse_param}, @dots{})
+Build a new parser object using the specified scanner.  There are no
+additional parameters unless @code{%parse-param}s are used.
+
+If the scanner is defined by @code{%code lexer}, this constructor is
+declared @code{protected} and is called automatically with a scanner
+created with the correct @code{%lex-param}s.
+@end deftypeop
+
+@deftypemethod {YYParser} {boolean} parse ()
+Run the syntactic analysis, and return @code{true} on success,
+@code{false} otherwise.
+@end deftypemethod
+
+@deftypemethod {YYParser} {boolean} recovering ()
+During the syntactic analysis, return @code{true} if recovering
+from a syntax error.
+@xref{Error Recovery}.
+@end deftypemethod
+
+@deftypemethod {YYParser} {java.io.PrintStream} getDebugStream ()
+@deftypemethodx {YYParser} {void} setDebugStream (java.io.printStream @var{o})
+Get or set the stream used for tracing the parsing.  It defaults to
+@code{System.err}.
+@end deftypemethod
+
+@deftypemethod {YYParser} {int} getDebugLevel ()
+@deftypemethodx {YYParser} {void} setDebugLevel (int @var{l})
+Get or set the tracing level.  Currently its value is either 0, no trace,
+or nonzero, full tracing.
+@end deftypemethod
+
+
+@node Java Scanner Interface
+@subsection Java Scanner Interface
+@c - %code lexer
+@c - %lex-param
+@c - Lexer interface
+
+There are two possible ways to interface a Bison-generated Java parser
+with a scanner: the scanner may be defined by @code{%code lexer}, or
+defined elsewhere.  In either case, the scanner has to implement the
+@code{Lexer} inner interface of the parser class.
+
+In the first case, the body of the scanner class is placed in
+@code{%code lexer} blocks.  If you want to pass parameters from the
+parser constructor to the scanner constructor, specify them with
+@code{%lex-param}; they are passed before @code{%parse-param}s to the
+constructor.
+
+In the second case, the scanner has to implement the @code{Lexer} interface,
+which is defined within the parser class (e.g., @code{YYParser.Lexer}).
+The constructor of the parser object will then accept an object
+implementing the interface; @code{%lex-param} is not used in this
+case.
+
+In both cases, the scanner has to implement the following methods.
+
+@deftypemethod {Lexer} {void} yyerror (Location @var{loc}, String @var{msg})
+This method is defined by the user to emit an error message.  The first
+parameter is omitted if location tracking is not active.  Its type can be
+changed using @code{%define location_type "@var{class-name}".}
+@end deftypemethod
+
+@deftypemethod {Lexer} {int} yylex ()
+Return the next token.  Its type is the return value, its semantic
+value and location are saved and returned by the ther methods in the
+interface.
+
+Use @code{%define lex_throws} to specify any uncaught exceptions.
+Default is @code{java.io.IOException}.
+@end deftypemethod
+
+@deftypemethod {Lexer} {Position} getStartPos ()
+@deftypemethodx {Lexer} {Position} getEndPos ()
+Return respectively the first position of the last token that
+@code{yylex} returned, and the first position beyond it.  These
+methods are not needed unless location tracking is active.
+
+The return type can be changed using @code{%define position_type
+"@var{class-name}".}
+@end deftypemethod
+
+@deftypemethod {Lexer} {Object} getLVal ()
+Return the semantical value of the last token that yylex returned.
+
+The return type can be changed using @code{%define stype
+"@var{class-name}".}
+@end deftypemethod
+
+
+@node Java Action Features
+@subsection Special Features for Use in Java Actions
+
+The following special constructs can be uses in Java actions.
+Other analogous C action features are currently unavailable for Java.
+
+Use @code{%define throws} to specify any uncaught exceptions from parser
+actions, and initial actions specified by @code{%initial-action}.
+
+@defvar $@var{n}
+The semantic value for the @var{n}th component of the current rule.
+This may not be assigned to.
+@xref{Java Semantic Values}.
+@end defvar
+
+@defvar $<@var{typealt}>@var{n}
+Like @code{$@var{n}} but specifies a alternative type @var{typealt}.
+@xref{Java Semantic Values}.
+@end defvar
+
+@defvar $$
+The semantic value for the grouping made by the current rule.  As a
+value, this is in the base type (@code{Object} or as specified by
+@code{%define stype}) as in not cast to the declared subtype because
+casts are not allowed on the left-hand side of Java assignments.
+Use an explicit Java cast if the correct subtype is needed.
+@xref{Java Semantic Values}.
+@end defvar
+
+@defvar $<@var{typealt}>$
+Same as @code{$$} since Java always allow assigning to the base type.
+Perhaps we should use this and @code{$<>$} for the value and @code{$$}
+for setting the value but there is currently no easy way to distinguish
+these constructs.
+@xref{Java Semantic Values}.
+@end defvar
+
+@defvar @@@var{n}
+The location information of the @var{n}th component of the current rule.
+This may not be assigned to.
+@xref{Java Location Values}.
+@end defvar
+
+@defvar @@$
+The location information of the grouping made by the current rule.
+@xref{Java Location Values}.
+@end defvar
+
+@deffn {Statement} {return YYABORT;}
+Return immediately from the parser, indicating failure.
+@xref{Java Parser Interface}.
+@end deffn
+
+@deffn {Statement} {return YYACCEPT;}
+Return immediately from the parser, indicating success.
+@xref{Java Parser Interface}.
+@end deffn
+
+@deffn {Statement} {return YYERROR;}
+Start error recovery without printing an error message. 
+@xref{Error Recovery}.
+@end deffn
+
+@deffn {Statement} {return YYFAIL;}
+Print an error message and start error recovery. 
+@xref{Error Recovery}.
+@end deffn
+
+@deftypefn {Function} {boolean} recovering ()
+Return whether error recovery is being done. In this state, the parser
+reads token until it reaches a known state, and then restarts normal
+operation.
+@xref{Error Recovery}.
+@end deftypefn
+
+@deftypefn  {Function} {protected void} yyerror (String msg)
+@deftypefnx {Function} {protected void} yyerror (Position pos, String msg)
+@deftypefnx {Function} {protected void} yyerror (Location loc, String msg)
+Print an error message using the @code{yyerror} method of the scanner
+instance in use.
+@end deftypefn
+
+
+@node Java Differences
+@subsection Differences between C/C++ and Java Grammars
+
+The different structure of the Java language forces several differences
+between C/C++ grammars, and grammars designed for Java parsers.  This
+section summarizes these differences.
+
+@itemize
+@item
+Java lacks a preprocessor, so the @code{YYERROR}, @code{YYACCEPT},
+@code{YYABORT} symbols (@pxref{Table of Symbols}) cannot obviously be
+macros.  Instead, they should be preceded by @code{return} when they
+appear in an action.  The actual definition of these symbols is
+opaque to the Bison grammar, and it might change in the future.  The
+only meaningful operation that you can do, is to return them.
+See @pxref{Java Action Features}.
+
+Note that of these three symbols, only @code{YYACCEPT} and
+@code{YYABORT} will cause a return from the @code{yyparse}
+method@footnote{Java parsers include the actions in a separate
+method than @code{yyparse} in order to have an intuitive syntax that
+corresponds to these C macros.}.
+
+@item
+Java lacks unions, so @code{%union} has no effect.  Instead, semantic
+values have a common base type: @code{Object} or as specified by
+@code{%define stype}.  Angle backets on @code{%token}, @code{type},
+@code{$@var{n}} and @code{$$} specify subtypes rather than fields of
+an union.  The type of @code{$$}, even with angle brackets, is the base
+type since Java casts are not allow on the left-hand side of assignments.
+Also, @code{$@var{n}} and @code{@@@var{n}} are not allowed on the
+left-hand side of assignments. See @pxref{Java Semantic Values} and
+@pxref{Java Action Features}.
+
+@item
+The prolog declarations have a different meaning than in C/C++ code.
+@table @asis
+@item @code{%code imports}
+blocks are placed at the beginning of the Java source code.  They may
+include copyright notices.  For a @code{package} declarations, it is
+suggested to use @code{%define package} instead.
+
+@item unqualified @code{%code}
+blocks are placed inside the parser class.
+
+@item @code{%code lexer}
+blocks, if specified, should include the implementation of the
+scanner.  If there is no such block, the scanner can be any class
+that implements the appropriate interface (see @pxref{Java Scanner
+Interface}).
+@end table
+
+Other @code{%code} blocks are not supported in Java parsers.
+In particular, @code{%@{ @dots{} %@}} blocks should not be used
+and may give an error in future versions of Bison.
+
+The epilogue has the same meaning as in C/C++ code and it can
+be used to define other classes used by the parser @emph{outside}
+the parser class.
+@end itemize
+
+
+@node Java Declarations Summary
+@subsection Java Declarations Summary
+
+This summary only include declarations specific to Java or have special
+meaning when used in a Java parser.
+
+@deffn {Directive} {%language "Java"}
+Generate a Java class for the parser.
+@end deffn
+
+@deffn {Directive} %lex-param @{@var{type} @var{name}@}
+A parameter for the lexer class defined by @code{%code lexer}
+@emph{only}, added as parameters to the lexer constructor and the parser
+constructor that @emph{creates} a lexer.  Default is none.
+@xref{Java Scanner Interface}.
+@end deffn
+
+@deffn {Directive} %name-prefix "@var{prefix}"
+The prefix of the parser class name @code{@var{prefix}Parser} if
+@code{%define parser_class_name} is not used.  Default is @code{YY}.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} %parse-param @{@var{type} @var{name}@}
+A parameter for the parser class added as parameters to constructor(s)
+and as fields initialized by the constructor(s).  Default is none.
+@xref{Java Parser Interface}.
+@end deffn
+
+@deffn {Directive} %token <@var{type}> @var{token} @dots{}
+Declare tokens.  Note that the angle brackets enclose a Java @emph{type}.
+@xref{Java Semantic Values}.
+@end deffn
+
+@deffn {Directive} %type <@var{type}> @var{nonterminal} @dots{}
+Declare the type of nonterminals.  Note that the angle brackets enclose
+a Java @emph{type}.
+@xref{Java Semantic Values}.
+@end deffn
+
+@deffn {Directive} %code @{ @var{code} @dots{} @}
+Code appended to the inside of the parser class.
+@xref{Java Differences}.
+@end deffn
+
+@deffn {Directive} {%code imports} @{ @var{code} @dots{} @}
+Code inserted just after the @code{package} declaration.
+@xref{Java Differences}.
+@end deffn
+
+@deffn {Directive} {%code lexer} @{ @var{code} @dots{} @}
+Code added to the body of a inner lexer class within the parser class.
+@xref{Java Scanner Interface}.
+@end deffn
+
+@deffn {Directive} %% @var{code} @dots{}
+Code (after the second @code{%%}) appended to the end of the file,
+@emph{outside} the parser class.
+@xref{Java Differences}.
+@end deffn
+
+@deffn {Directive} %@{ @var{code} @dots{} %@}
+Not supported.  Use @code{%code import} instead.
+@xref{Java Differences}.
+@end deffn
+
+@deffn {Directive} {%define abstract}
+Whether the parser class is declared @code{abstract}.  Default is false.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define extends} "@var{superclass}"
+The superclass of the parser class.  Default is none.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define final}
+Whether the parser class is declared @code{final}.  Default is false.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define implements} "@var{interfaces}"
+The implemented interfaces of the parser class, a comma-separated list.
+Default is none.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define lex_throws} "@var{exceptions}"
+The exceptions thrown by the @code{yylex} method of the lexer, a
+comma-separated list.  Default is @code{java.io.IOException}.
+@xref{Java Scanner Interface}.
+@end deffn
+
+@deffn {Directive} {%define location_type} "@var{class}"
+The name of the class used for locations (a range between two
+positions).  This class is generated as an inner class of the parser
+class by @command{bison}.  Default is @code{Location}.
+@xref{Java Location Values}.
+@end deffn
+
+@deffn {Directive} {%define package} "@var{package}"
+The package to put the parser class in.  Default is none.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define parser_class_name} "@var{name}"
+The name of the parser class.  Default is @code{YYParser} or
+@code{@var{name-prefix}Parser}.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define position_type} "@var{class}"
+The name of the class used for positions. This class must be supplied by
+the user.  Default is @code{Position}.
+@xref{Java Location Values}.
+@end deffn
+
+@deffn {Directive} {%define public}
+Whether the parser class is declared @code{public}.  Default is false.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define stype} "@var{class}"
+The base type of semantic values.  Default is @code{Object}.
+@xref{Java Semantic Values}.
+@end deffn
+
+@deffn {Directive} {%define strictfp}
+Whether the parser class is declared @code{strictfp}.  Default is false.
+@xref{Java Bison Interface}.
+@end deffn
+
+@deffn {Directive} {%define throws} "@var{exceptions}"
+The exceptions thrown by user-supplied parser actions and
+@code{%initial-action}, a comma-separated list.  Default is none.
+@xref{Java Parser Interface}.
+@end deffn
+
+
+@c ================================================= FAQ
+
+@node FAQ
+@chapter Frequently Asked Questions
+@cindex frequently asked questions
+@cindex questions
+
+Several questions about Bison come up occasionally.  Here some of them
+are addressed.
+
+@menu
+* Memory Exhausted::            Breaking the Stack Limits
+* How Can I Reset the Parser::  @code{yyparse} Keeps some State
+* Strings are Destroyed::       @code{yylval} Loses Track of Strings
+* Implementing Gotos/Loops::    Control Flow in the Calculator
+* Multiple start-symbols::      Factoring closely related grammars
+* Secure?  Conform?::           Is Bison @acronym{POSIX} safe?
+* I can't build Bison::         Troubleshooting
+* Where can I find help?::      Troubleshouting
+* Bug Reports::                 Troublereporting
+* More Languages::              Parsers in C++, Java, and so on
+* Beta Testing::                Experimenting development versions
+* Mailing Lists::               Meeting other Bison users
+@end menu
+
+@node Memory Exhausted
+@section Memory Exhausted
+
+@display
+My parser returns with error with a @samp{memory exhausted}
+message.  What can I do?
+@end display
+
+This question is already addressed elsewhere, @xref{Recursion,
+,Recursive Rules}.
+
+@node How Can I Reset the Parser
+@section How Can I Reset the Parser
+
+The following phenomenon has several symptoms, resulting in the
+following typical questions:
+
+@display
+I invoke @code{yyparse} several times, and on correct input it works
+properly; but when a parse error is found, all the other calls fail
+too.  How can I reset the error flag of @code{yyparse}?
+@end display
+
+@noindent
+or
+
+@display
+My parser includes support for an @samp{#include}-like feature, in
+which case I run @code{yyparse} from @code{yyparse}.  This fails
+although I did specify @code{%define api.pure}.
+@end display
+
+These problems typically come not from Bison itself, but from
+Lex-generated scanners.  Because these scanners use large buffers for
+speed, they might not notice a change of input file.  As a
+demonstration, consider the following source file,
+@file{first-line.l}:
+
+@verbatim
+%{
+#include <stdio.h>
+#include <stdlib.h>
+%}
+%%
+.*\n    ECHO; return 1;
+%%
+int
+yyparse (char const *file)
+{
+  yyin = fopen (file, "r");
+  if (!yyin)
+    exit (2);
+  /* One token only.  */
+  yylex ();
+  if (fclose (yyin) != 0)
+    exit (3);
+  return 0;
+}
+
+int
+main (void)
+{
+  yyparse ("input");
+  yyparse ("input");
+  return 0;
+}
+@end verbatim
+
+@noindent
+If the file @file{input} contains
+
+@verbatim
+input:1: Hello,
+input:2: World!
+@end verbatim
+
+@noindent
+then instead of getting the first line twice, you get:
+
+@example
+$ @kbd{flex -ofirst-line.c first-line.l}
+$ @kbd{gcc  -ofirst-line   first-line.c -ll}
+$ @kbd{./first-line}
+input:1: Hello,
+input:2: World!
+@end example
+
+Therefore, whenever you change @code{yyin}, you must tell the
+Lex-generated scanner to discard its current buffer and switch to the
+new one.  This depends upon your implementation of Lex; see its
+documentation for more.  For Flex, it suffices to call
+@samp{YY_FLUSH_BUFFER} after each change to @code{yyin}.  If your
+Flex-generated scanner needs to read from several input streams to
+handle features like include files, you might consider using Flex
+functions like @samp{yy_switch_to_buffer} that manipulate multiple
+input buffers.
+
+If your Flex-generated scanner uses start conditions (@pxref{Start
+conditions, , Start conditions, flex, The Flex Manual}), you might
+also want to reset the scanner's state, i.e., go back to the initial
+start condition, through a call to @samp{BEGIN (0)}.
+
+@node Strings are Destroyed
+@section Strings are Destroyed
+
+@display
+My parser seems to destroy old strings, or maybe it loses track of
+them.  Instead of reporting @samp{"foo", "bar"}, it reports
+@samp{"bar", "bar"}, or even @samp{"foo\nbar", "bar"}.
+@end display
+
+This error is probably the single most frequent ``bug report'' sent to
+Bison lists, but is only concerned with a misunderstanding of the role
+of the scanner.  Consider the following Lex code:
+
+@verbatim
+%{
+#include <stdio.h>
+char *yylval = NULL;
+%}
+%%
+.*    yylval = yytext; return 1;
+\n    /* IGNORE */
+%%
+int
+main ()
+{
+  /* Similar to using $1, $2 in a Bison action.  */
+  char *fst = (yylex (), yylval);
+  char *snd = (yylex (), yylval);
+  printf ("\"%s\", \"%s\"\n", fst, snd);
+  return 0;
+}
+@end verbatim
+
+If you compile and run this code, you get:
+
+@example
+$ @kbd{flex -osplit-lines.c split-lines.l}
+$ @kbd{gcc  -osplit-lines   split-lines.c -ll}
+$ @kbd{printf 'one\ntwo\n' | ./split-lines}
+"one
+two", "two"
+@end example
+
+@noindent
+this is because @code{yytext} is a buffer provided for @emph{reading}
+in the action, but if you want to keep it, you have to duplicate it
+(e.g., using @code{strdup}).  Note that the output may depend on how
+your implementation of Lex handles @code{yytext}.  For instance, when
+given the Lex compatibility option @option{-l} (which triggers the
+option @samp{%array}) Flex generates a different behavior:
+
+@example
+$ @kbd{flex -l -osplit-lines.c split-lines.l}
+$ @kbd{gcc     -osplit-lines   split-lines.c -ll}
+$ @kbd{printf 'one\ntwo\n' | ./split-lines}
+"two", "two"
+@end example
+
+
+@node Implementing Gotos/Loops
+@section Implementing Gotos/Loops
+
+@display
+My simple calculator supports variables, assignments, and functions,
+but how can I implement gotos, or loops?
+@end display
+
+Although very pedagogical, the examples included in the document blur
+the distinction to make between the parser---whose job is to recover
+the structure of a text and to transmit it to subsequent modules of
+the program---and the processing (such as the execution) of this
+structure.  This works well with so called straight line programs,
+i.e., precisely those that have a straightforward execution model:
+execute simple instructions one after the others.
+
+@cindex abstract syntax tree
+@cindex @acronym{AST}
+If you want a richer model, you will probably need to use the parser
+to construct a tree that does represent the structure it has
+recovered; this tree is usually called the @dfn{abstract syntax tree},
+or @dfn{@acronym{AST}} for short.  Then, walking through this tree,
+traversing it in various ways, will enable treatments such as its
+execution or its translation, which will result in an interpreter or a
+compiler.
+
+This topic is way beyond the scope of this manual, and the reader is
+invited to consult the dedicated literature.
+
+
+@node Multiple start-symbols
+@section Multiple start-symbols
+
+@display
+I have several closely related grammars, and I would like to share their
+implementations.  In fact, I could use a single grammar but with
+multiple entry points.
+@end display
+
+Bison does not support multiple start-symbols, but there is a very
+simple means to simulate them.  If @code{foo} and @code{bar} are the two
+pseudo start-symbols, then introduce two new tokens, say
+@code{START_FOO} and @code{START_BAR}, and use them as switches from the
+real start-symbol:
+
+@example
+%token START_FOO START_BAR;
+%start start;
+start: START_FOO foo
+     | START_BAR bar;
+@end example
+
+These tokens prevents the introduction of new conflicts.  As far as the
+parser goes, that is all that is needed.
+
+Now the difficult part is ensuring that the scanner will send these
+tokens first.  If your scanner is hand-written, that should be
+straightforward.  If your scanner is generated by Lex, them there is
+simple means to do it: recall that anything between @samp{%@{ ... %@}}
+after the first @code{%%} is copied verbatim in the top of the generated
+@code{yylex} function.  Make sure a variable @code{start_token} is
+available in the scanner (e.g., a global variable or using
+@code{%lex-param} etc.), and use the following:
+
+@example
+  /* @r{Prologue.}  */
+%%
+%@{
+  if (start_token)
+    @{
+      int t = start_token;
+      start_token = 0;
+      return t;
+    @}
+%@}
+  /* @r{The rules.}  */
+@end example
+
+
+@node Secure?  Conform?
+@section Secure?  Conform?
+
+@display
+Is Bison secure?  Does it conform to POSIX?
+@end display
+
+If you're looking for a guarantee or certification, we don't provide it.
+However, Bison is intended to be a reliable program that conforms to the
+@acronym{POSIX} specification for Yacc.  If you run into problems,
+please send us a bug report.
+
+@node I can't build Bison
+@section I can't build Bison
+
+@display
+I can't build Bison because @command{make} complains that
+@code{msgfmt} is not found.
+What should I do?
+@end display
+
+Like most GNU packages with internationalization support, that feature
+is turned on by default.  If you have problems building in the @file{po}
+subdirectory, it indicates that your system's internationalization
+support is lacking.  You can re-configure Bison with
+@option{--disable-nls} to turn off this support, or you can install GNU
+gettext from @url{ftp://ftp.gnu.org/gnu/gettext/} and re-configure
+Bison.  See the file @file{ABOUT-NLS} for more information.
+
+
+@node Where can I find help?
+@section Where can I find help?
+
+@display
+I'm having trouble using Bison.  Where can I find help?
+@end display
+
+First, read this fine manual.  Beyond that, you can send mail to
+@email{help-bison@@gnu.org}.  This mailing list is intended to be
+populated with people who are willing to answer questions about using
+and installing Bison.  Please keep in mind that (most of) the people on
+the list have aspects of their lives which are not related to Bison (!),
+so you may not receive an answer to your question right away.  This can
+be frustrating, but please try not to honk them off; remember that any
+help they provide is purely voluntary and out of the kindness of their
+hearts.
+
+@node Bug Reports
+@section Bug Reports
+
+@display
+I found a bug.  What should I include in the bug report?
+@end display
+
+Before you send a bug report, make sure you are using the latest
+version.  Check @url{ftp://ftp.gnu.org/pub/gnu/bison/} or one of its
+mirrors.  Be sure to include the version number in your bug report.  If
+the bug is present in the latest version but not in a previous version,
+try to determine the most recent version which did not contain the bug.
+
+If the bug is parser-related, you should include the smallest grammar
+you can which demonstrates the bug.  The grammar file should also be
+complete (i.e., I should be able to run it through Bison without having
+to edit or add anything).  The smaller and simpler the grammar, the
+easier it will be to fix the bug.
+
+Include information about your compilation environment, including your
+operating system's name and version and your compiler's name and
+version.  If you have trouble compiling, you should also include a
+transcript of the build session, starting with the invocation of
+`configure'.  Depending on the nature of the bug, you may be asked to
+send additional files as well (such as `config.h' or `config.cache').
+
+Patches are most welcome, but not required.  That is, do not hesitate to
+send a bug report just because you can not provide a fix.
+
+Send bug reports to @email{bug-bison@@gnu.org}.
+
+@node More Languages
+@section More Languages
+
+@display
+Will Bison ever have C++ and Java support?  How about @var{insert your
+favorite language here}?
+@end display
+
+C++ and Java support is there now, and is documented.  We'd love to add other
+languages; contributions are welcome.
+
+@node Beta Testing
+@section Beta Testing
+
+@display
+What is involved in being a beta tester?
+@end display
+
+It's not terribly involved.  Basically, you would download a test
+release, compile it, and use it to build and run a parser or two.  After
+that, you would submit either a bug report or a message saying that
+everything is okay.  It is important to report successes as well as
+failures because test releases eventually become mainstream releases,
+but only if they are adequately tested.  If no one tests, development is
+essentially halted.
+
+Beta testers are particularly needed for operating systems to which the
+developers do not have easy access.  They currently have easy access to
+recent GNU/Linux and Solaris versions.  Reports about other operating
+systems are especially welcome.
+
+@node Mailing Lists
+@section Mailing Lists
+
+@display
+How do I join the help-bison and bug-bison mailing lists?
+@end display
+
+See @url{http://lists.gnu.org/}.
+
+@c ================================================= Table of Symbols
+
+@node Table of Symbols
+@appendix Bison Symbols
+@cindex Bison symbols, table of
+@cindex symbols in Bison, table of
+
+@deffn {Variable} @@$
+In an action, the location of the left-hand side of the rule.
+@xref{Locations, , Locations Overview}.
+@end deffn
+
+@deffn {Variable} @@@var{n}
+In an action, the location of the @var{n}-th symbol of the right-hand
+side of the rule.  @xref{Locations, , Locations Overview}.
+@end deffn
+
+@deffn {Variable} $$
+In an action, the semantic value of the left-hand side of the rule.
+@xref{Actions}.
+@end deffn
+
+@deffn {Variable} $@var{n}
+In an action, the semantic value of the @var{n}-th symbol of the
+right-hand side of the rule.  @xref{Actions}.
+@end deffn
+
+@deffn {Delimiter} %%
+Delimiter used to separate the grammar rule section from the
+Bison declarations section or the epilogue.
+@xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}.
+@end deffn
+
+@c Don't insert spaces, or check the DVI output.
+@deffn {Delimiter} %@{@var{code}%@}
+All code listed between @samp{%@{} and @samp{%@}} is copied directly to
+the output file uninterpreted.  Such code forms the prologue of the input
+file.  @xref{Grammar Outline, ,Outline of a Bison
+Grammar}.
+@end deffn
+
+@deffn {Construct} /*@dots{}*/
+Comment delimiters, as in C.
+@end deffn
+
+@deffn {Delimiter} :
+Separates a rule's result from its components.  @xref{Rules, ,Syntax of
+Grammar Rules}.
+@end deffn
+
+@deffn {Delimiter} ;
+Terminates a rule.  @xref{Rules, ,Syntax of Grammar Rules}.
+@end deffn
+
+@deffn {Delimiter} |
+Separates alternate rules for the same result nonterminal.
+@xref{Rules, ,Syntax of Grammar Rules}.
+@end deffn
+
+@deffn {Directive} <*>
+Used to define a default tagged @code{%destructor} or default tagged
+@code{%printer}.
+
+This feature is experimental.
+More user feedback will help to determine whether it should become a permanent
+feature.
+
+@xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
+
+@deffn {Directive} <>
+Used to define a default tagless @code{%destructor} or default tagless
+@code{%printer}.
+
+This feature is experimental.
+More user feedback will help to determine whether it should become a permanent
+feature.
+
+@xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
+
+@deffn {Symbol} $accept
+The predefined nonterminal whose only rule is @samp{$accept: @var{start}
+$end}, where @var{start} is the start symbol.  @xref{Start Decl, , The
+Start-Symbol}.  It cannot be used in the grammar.
+@end deffn
+
+@deffn {Directive} %code @{@var{code}@}
+@deffnx {Directive} %code @var{qualifier} @{@var{code}@}
+Insert @var{code} verbatim into output parser source.
+@xref{Decl Summary,,%code}.
+@end deffn
+
+@deffn {Directive} %debug
+Equip the parser for debugging.  @xref{Decl Summary}.
+@end deffn
+
+@deffn {Directive} %debug
+Equip the parser for debugging.  @xref{Decl Summary}.
+@end deffn
+
+@ifset defaultprec
+@deffn {Directive} %default-prec
+Assign a precedence to rules that lack an explicit @samp{%prec}
+modifier.  @xref{Contextual Precedence, ,Context-Dependent
+Precedence}.
+@end deffn
+@end ifset
+
+@deffn {Directive} %define @var{define-variable}
+@deffnx {Directive} %define @var{define-variable} @var{value}
+Define a variable to adjust Bison's behavior.
+@xref{Decl Summary,,%define}.
+@end deffn
+
+@deffn {Directive} %defines
+Bison declaration to create a header file meant for the scanner.
+@xref{Decl Summary}.
+@end deffn
+
+@deffn {Directive} %defines @var{defines-file}
+Same as above, but save in the file @var{defines-file}.
+@xref{Decl Summary}.
+@end deffn
+
+@deffn {Directive} %destructor
+Specify how the parser should reclaim the memory associated to
+discarded symbols.  @xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
+
+@deffn {Directive} %dprec
+Bison declaration to assign a precedence to a rule that is used at parse
+time to resolve reduce/reduce conflicts.  @xref{GLR Parsers, ,Writing
+@acronym{GLR} Parsers}.
+@end deffn
+
+@deffn {Symbol} $end
+The predefined token marking the end of the token stream.  It cannot be
+used in the grammar.
+@end deffn
+
+@deffn {Symbol} error
+A token name reserved for error recovery.  This token may be used in
+grammar rules so as to allow the Bison parser to recognize an error in
+the grammar without halting the process.  In effect, a sentence
+containing an error may be recognized as valid.  On a syntax error, the
+token @code{error} becomes the current lookahead token.  Actions
+corresponding to @code{error} are then executed, and the lookahead
+token is reset to the token that originally caused the violation.
+@xref{Error Recovery}.
+@end deffn
+
+@deffn {Directive} %error-verbose
+Bison declaration to request verbose, specific error message strings
+when @code{yyerror} is called.
+@end deffn
+
+@deffn {Directive} %file-prefix "@var{prefix}"
+Bison declaration to set the prefix of the output files.  @xref{Decl
+Summary}.
+@end deffn
+
+@deffn {Directive} %glr-parser
+Bison declaration to produce a @acronym{GLR} parser.  @xref{GLR
+Parsers, ,Writing @acronym{GLR} Parsers}.
+@end deffn
+
+@deffn {Directive} %initial-action
+Run user code before parsing.  @xref{Initial Action Decl, , Performing Actions before Parsing}.
+@end deffn
+
+@deffn {Directive} %language
+Specify the programming language for the generated parser.
+@xref{Decl Summary}.
+@end deffn
+
+@deffn {Directive} %left
+Bison declaration to assign left associativity to token(s).
+@xref{Precedence Decl, ,Operator Precedence}.
+@end deffn
+
+@deffn {Directive} %lex-param @{@var{argument-declaration}@}
+Bison declaration to specifying an additional parameter that
+@code{yylex} should accept.  @xref{Pure Calling,, Calling Conventions
+for Pure Parsers}.
+@end deffn
+
+@deffn {Directive} %merge
+Bison declaration to assign a merging function to a rule.  If there is a
+reduce/reduce conflict with a rule having the same merging function, the
+function is applied to the two semantic values to get a single result.
+@xref{GLR Parsers, ,Writing @acronym{GLR} Parsers}.
+@end deffn
+
+@deffn {Directive} %name-prefix "@var{prefix}"
+Bison declaration to rename the external symbols.  @xref{Decl Summary}.
+@end deffn
+
+@ifset defaultprec
+@deffn {Directive} %no-default-prec
+Do not assign a precedence to rules that lack an explicit @samp{%prec}
+modifier.  @xref{Contextual Precedence, ,Context-Dependent
+Precedence}.
+@end deffn
+@end ifset
+
+@deffn {Directive} %no-lines
+Bison declaration to avoid generating @code{#line} directives in the
+parser file.  @xref{Decl Summary}.
+@end deffn
+
+@deffn {Directive} %nonassoc
+Bison declaration to assign nonassociativity to token(s).
+@xref{Precedence Decl, ,Operator Precedence}.
+@end deffn
+
+@deffn {Directive} %output "@var{file}"
+Bison declaration to set the name of the parser file.  @xref{Decl
+Summary}.
+@end deffn
+
+@deffn {Directive} %parse-param @{@var{argument-declaration}@}
+Bison declaration to specifying an additional parameter that
+@code{yyparse} should accept.  @xref{Parser Function,, The Parser
+Function @code{yyparse}}.
+@end deffn
+
+@deffn {Directive} %prec
+Bison declaration to assign a precedence to a specific rule.
+@xref{Contextual Precedence, ,Context-Dependent Precedence}.
+@end deffn
+
+@deffn {Directive} %pure-parser
+Deprecated version of @code{%define api.pure} (@pxref{Decl Summary, ,%define}),
+for which Bison is more careful to warn about unreasonable usage.
+@end deffn
+
+@deffn {Directive} %require "@var{version}"
+Require version @var{version} or higher of Bison.  @xref{Require Decl, ,
+Require a Version of Bison}.
+@end deffn
+
+@deffn {Directive} %right
+Bison declaration to assign right associativity to token(s).
+@xref{Precedence Decl, ,Operator Precedence}.
+@end deffn
+
+@deffn {Directive} %skeleton
+Specify the skeleton to use; usually for development.
+@xref{Decl Summary}.
+@end deffn
+
+@deffn {Directive} %start
+Bison declaration to specify the start symbol.  @xref{Start Decl, ,The
+Start-Symbol}.
+@end deffn
+
+@deffn {Directive} %token
+Bison declaration to declare token(s) without specifying precedence.
+@xref{Token Decl, ,Token Type Names}.
+@end deffn
+
+@deffn {Directive} %token-table
+Bison declaration to include a token name table in the parser file.
+@xref{Decl Summary}.
+@end deffn
+
+@deffn {Directive} %type
+Bison declaration to declare nonterminals.  @xref{Type Decl,
+,Nonterminal Symbols}.
+@end deffn
+
+@deffn {Symbol} $undefined
+The predefined token onto which all undefined values returned by
+@code{yylex} are mapped.  It cannot be used in the grammar, rather, use
+@code{error}.
+@end deffn
+
+@deffn {Directive} %union
+Bison declaration to specify several possible data types for semantic
+values.  @xref{Union Decl, ,The Collection of Value Types}.
+@end deffn
+
+@deffn {Macro} YYABORT
+Macro to pretend that an unrecoverable syntax error has occurred, by
+making @code{yyparse} return 1 immediately.  The error reporting
+function @code{yyerror} is not called.  @xref{Parser Function, ,The
+Parser Function @code{yyparse}}.
+
+For Java parsers, this functionality is invoked using @code{return YYABORT;}
+instead.
+@end deffn
+
+@deffn {Macro} YYACCEPT
+Macro to pretend that a complete utterance of the language has been
+read, by making @code{yyparse} return 0 immediately.
+@xref{Parser Function, ,The Parser Function @code{yyparse}}.
+
+For Java parsers, this functionality is invoked using @code{return YYACCEPT;}
+instead.
+@end deffn
+
+@deffn {Macro} YYBACKUP
+Macro to discard a value from the parser stack and fake a lookahead
+token.  @xref{Action Features, ,Special Features for Use in Actions}.
+@end deffn
+
+@deffn {Variable} yychar
+External integer variable that contains the integer value of the
+lookahead token.  (In a pure parser, it is a local variable within
+@code{yyparse}.)  Error-recovery rule actions may examine this variable.
+@xref{Action Features, ,Special Features for Use in Actions}.
+@end deffn
+
+@deffn {Variable} yyclearin
+Macro used in error-recovery rule actions.  It clears the previous
+lookahead token.  @xref{Error Recovery}.
+@end deffn
+
+@deffn {Macro} YYDEBUG
+Macro to define to equip the parser with tracing code.  @xref{Tracing,
+,Tracing Your Parser}.
+@end deffn
+
+@deffn {Variable} yydebug
+External integer variable set to zero by default.  If @code{yydebug}
+is given a nonzero value, the parser will output information on input
+symbols and parser action.  @xref{Tracing, ,Tracing Your Parser}.
+@end deffn
+
+@deffn {Macro} yyerrok
+Macro to cause parser to recover immediately to its normal mode
+after a syntax error.  @xref{Error Recovery}.
+@end deffn
+
+@deffn {Macro} YYERROR
+Macro to pretend that a syntax error has just been detected: call
+@code{yyerror} and then perform normal error recovery if possible
+(@pxref{Error Recovery}), or (if recovery is impossible) make
+@code{yyparse} return 1.  @xref{Error Recovery}.
+
+For Java parsers, this functionality is invoked using @code{return YYERROR;}
+instead.
+@end deffn
+
+@deffn {Function} yyerror
+User-supplied function to be called by @code{yyparse} on error.
+@xref{Error Reporting, ,The Error
+Reporting Function @code{yyerror}}.
+@end deffn
+
+@deffn {Macro} YYERROR_VERBOSE
+An obsolete macro that you define with @code{#define} in the prologue
+to request verbose, specific error message strings
+when @code{yyerror} is called.  It doesn't matter what definition you
+use for @code{YYERROR_VERBOSE}, just whether you define it.  Using
+@code{%error-verbose} is preferred.
+@end deffn
+
+@deffn {Macro} YYINITDEPTH
+Macro for specifying the initial size of the parser stack.
+@xref{Memory Management}.
+@end deffn
+
+@deffn {Function} yylex
+User-supplied lexical analyzer function, called with no arguments to get
+the next token.  @xref{Lexical, ,The Lexical Analyzer Function
+@code{yylex}}.
+@end deffn
+
+@deffn {Macro} YYLEX_PARAM
+An obsolete macro for specifying an extra argument (or list of extra
+arguments) for @code{yyparse} to pass to @code{yylex}.  The use of this
+macro is deprecated, and is supported only for Yacc like parsers.
+@xref{Pure Calling,, Calling Conventions for Pure Parsers}.
+@end deffn
+
+@deffn {Variable} yylloc
+External variable in which @code{yylex} should place the line and column
+numbers associated with a token.  (In a pure parser, it is a local
+variable within @code{yyparse}, and its address is passed to
+@code{yylex}.)
+You can ignore this variable if you don't use the @samp{@@} feature in the
+grammar actions.
+@xref{Token Locations, ,Textual Locations of Tokens}.
+In semantic actions, it stores the location of the lookahead token.
+@xref{Actions and Locations, ,Actions and Locations}.
+@end deffn
+
+@deffn {Type} YYLTYPE
+Data type of @code{yylloc}; by default, a structure with four
+members.  @xref{Location Type, , Data Types of Locations}.
+@end deffn
+
+@deffn {Variable} yylval
+External variable in which @code{yylex} should place the semantic
+value associated with a token.  (In a pure parser, it is a local
+variable within @code{yyparse}, and its address is passed to
+@code{yylex}.)
+@xref{Token Values, ,Semantic Values of Tokens}.
+In semantic actions, it stores the semantic value of the lookahead token.
+@xref{Actions, ,Actions}.
+@end deffn
+
+@deffn {Macro} YYMAXDEPTH
+Macro for specifying the maximum size of the parser stack.  @xref{Memory
+Management}.
+@end deffn
+
+@deffn {Variable} yynerrs
+Global variable which Bison increments each time it reports a syntax error.
+(In a pure parser, it is a local variable within @code{yyparse}. In a
+pure push parser, it is a member of yypstate.)
+@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
+@end deffn
+
+@deffn {Function} yyparse
+The parser function produced by Bison; call this function to start
+parsing.  @xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@end deffn
+
+@deffn {Function} yypstate_delete
+The function to delete a parser instance, produced by Bison in push mode;
+call this function to delete the memory associated with a parser.
+@xref{Parser Delete Function, ,The Parser Delete Function
+@code{yypstate_delete}}.
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+@end deffn
+
+@deffn {Function} yypstate_new
+The function to create a parser instance, produced by Bison in push mode;
+call this function to create a new parser.
+@xref{Parser Create Function, ,The Parser Create Function
+@code{yypstate_new}}.
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+@end deffn
+
+@deffn {Function} yypull_parse
+The parser function produced by Bison in push mode; call this function to
+parse the rest of the input stream.
+@xref{Pull Parser Function, ,The Pull Parser Function
+@code{yypull_parse}}.
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+@end deffn
+
+@deffn {Function} yypush_parse
+The parser function produced by Bison in push mode; call this function to
+parse a single token.  @xref{Push Parser Function, ,The Push Parser Function
+@code{yypush_parse}}.
+(The current push parsing interface is experimental and may evolve.
+More user feedback will help to stabilize it.)
+@end deffn
+
+@deffn {Macro} YYPARSE_PARAM
+An obsolete macro for specifying the name of a parameter that
+@code{yyparse} should accept.  The use of this macro is deprecated, and
+is supported only for Yacc like parsers.  @xref{Pure Calling,, Calling
+Conventions for Pure Parsers}.
+@end deffn
+
+@deffn {Macro} YYRECOVERING
+The expression @code{YYRECOVERING ()} yields 1 when the parser
+is recovering from a syntax error, and 0 otherwise.
+@xref{Action Features, ,Special Features for Use in Actions}.
+@end deffn
+
+@deffn {Macro} YYSTACK_USE_ALLOCA
+Macro used to control the use of @code{alloca} when the C
+@acronym{LALR}(1) parser needs to extend its stacks.  If defined to 0,
+the parser will use @code{malloc} to extend its stacks.  If defined to
+1, the parser will use @code{alloca}.  Values other than 0 and 1 are
+reserved for future Bison extensions.  If not defined,
+@code{YYSTACK_USE_ALLOCA} defaults to 0.
+
+In the all-too-common case where your code may run on a host with a
+limited stack and with unreliable stack-overflow checking, you should
+set @code{YYMAXDEPTH} to a value that cannot possibly result in
+unchecked stack overflow on any of your target hosts when
+@code{alloca} is called.  You can inspect the code that Bison
+generates in order to determine the proper numeric values.  This will
+require some expertise in low-level implementation details.
+@end deffn
+
+@deffn {Type} YYSTYPE
+Data type of semantic values; @code{int} by default.
+@xref{Value Type, ,Data Types of Semantic Values}.
+@end deffn
+
+@node Glossary
+@appendix Glossary
+@cindex glossary
+
+@table @asis
+@item Backus-Naur Form (@acronym{BNF}; also called ``Backus Normal Form'')
+Formal method of specifying context-free grammars originally proposed
+by John Backus, and slightly improved by Peter Naur in his 1960-01-02
+committee document contributing to what became the Algol 60 report.
+@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
+
+@item Context-free grammars
+Grammars specified as rules that can be applied regardless of context.
+Thus, if there is a rule which says that an integer can be used as an
+expression, integers are allowed @emph{anywhere} an expression is
+permitted.  @xref{Language and Grammar, ,Languages and Context-Free
+Grammars}.
+
+@item Dynamic allocation
+Allocation of memory that occurs during execution, rather than at
+compile time or on entry to a function.
+
+@item Empty string
+Analogous to the empty set in set theory, the empty string is a
+character string of length zero.
+
+@item Finite-state stack machine
+A ``machine'' that has discrete states in which it is said to exist at
+each instant in time.  As input to the machine is processed, the
+machine moves from state to state as specified by the logic of the
+machine.  In the case of the parser, the input is the language being
+parsed, and the states correspond to various stages in the grammar
+rules.  @xref{Algorithm, ,The Bison Parser Algorithm}.
+
+@item Generalized @acronym{LR} (@acronym{GLR})
+A parsing algorithm that can handle all context-free grammars, including those
+that are not @acronym{LALR}(1).  It resolves situations that Bison's
+usual @acronym{LALR}(1)
+algorithm cannot by effectively splitting off multiple parsers, trying all
+possible parsers, and discarding those that fail in the light of additional
+right context.  @xref{Generalized LR Parsing, ,Generalized
+@acronym{LR} Parsing}.
+
+@item Grouping
+A language construct that is (in general) grammatically divisible;
+for example, `expression' or `declaration' in C@.
+@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
+
+@item Infix operator
+An arithmetic operator that is placed between the operands on which it
+performs some operation.
+
+@item Input stream
+A continuous flow of data between devices or programs.
+
+@item Language construct
+One of the typical usage schemas of the language.  For example, one of
+the constructs of the C language is the @code{if} statement.
+@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
+
+@item Left associativity
+Operators having left associativity are analyzed from left to right:
+@samp{a+b+c} first computes @samp{a+b} and then combines with
+@samp{c}.  @xref{Precedence, ,Operator Precedence}.
+
+@item Left recursion
+A rule whose result symbol is also its first component symbol; for
+example, @samp{expseq1 : expseq1 ',' exp;}.  @xref{Recursion, ,Recursive
+Rules}.
+
+@item Left-to-right parsing
+Parsing a sentence of a language by analyzing it token by token from
+left to right.  @xref{Algorithm, ,The Bison Parser Algorithm}.
+
+@item Lexical analyzer (scanner)
+A function that reads an input stream and returns tokens one by one.
+@xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+
+@item Lexical tie-in
+A flag, set by actions in the grammar rules, which alters the way
+tokens are parsed.  @xref{Lexical Tie-ins}.
+
+@item Literal string token
+A token which consists of two or more fixed characters.  @xref{Symbols}.
+
+@item Lookahead token
+A token already read but not yet shifted.  @xref{Lookahead, ,Lookahead
+Tokens}.
+
+@item @acronym{LALR}(1)
+The class of context-free grammars that Bison (like most other parser
+generators) can handle; a subset of @acronym{LR}(1).  @xref{Mystery
+Conflicts, ,Mysterious Reduce/Reduce Conflicts}.
+
+@item @acronym{LR}(1)
+The class of context-free grammars in which at most one token of
+lookahead is needed to disambiguate the parsing of any piece of input.
+
+@item Nonterminal symbol
+A grammar symbol standing for a grammatical construct that can
+be expressed through rules in terms of smaller constructs; in other
+words, a construct that is not a token.  @xref{Symbols}.
+
+@item Parser
+A function that recognizes valid sentences of a language by analyzing
+the syntax structure of a set of tokens passed to it from a lexical
+analyzer.
+
+@item Postfix operator
+An arithmetic operator that is placed after the operands upon which it
+performs some operation.
+
+@item Reduction
+Replacing a string of nonterminals and/or terminals with a single
+nonterminal, according to a grammar rule.  @xref{Algorithm, ,The Bison
+Parser Algorithm}.
+
+@item Reentrant
+A reentrant subprogram is a subprogram which can be in invoked any
+number of times in parallel, without interference between the various
+invocations.  @xref{Pure Decl, ,A Pure (Reentrant) Parser}.
+
+@item Reverse polish notation
+A language in which all operators are postfix operators.
+
+@item Right recursion
+A rule whose result symbol is also its last component symbol; for
+example, @samp{expseq1: exp ',' expseq1;}.  @xref{Recursion, ,Recursive
+Rules}.
+
+@item Semantics
+In computer languages, the semantics are specified by the actions
+taken for each instance of the language, i.e., the meaning of
+each statement.  @xref{Semantics, ,Defining Language Semantics}.
+
+@item Shift
+A parser is said to shift when it makes the choice of analyzing
+further input from the stream rather than reducing immediately some
+already-recognized rule.  @xref{Algorithm, ,The Bison Parser Algorithm}.
+
+@item Single-character literal
+A single character that is recognized and interpreted as is.
+@xref{Grammar in Bison, ,From Formal Rules to Bison Input}.
+
+@item Start symbol
+The nonterminal symbol that stands for a complete valid utterance in
+the language being parsed.  The start symbol is usually listed as the
+first nonterminal symbol in a language specification.
+@xref{Start Decl, ,The Start-Symbol}.
+
+@item Symbol table
+A data structure where symbol names and associated data are stored
+during parsing to allow for recognition and use of existing
+information in repeated uses of a symbol.  @xref{Multi-function Calc}.
+
+@item Syntax error
+An error encountered during parsing of an input stream due to invalid
+syntax.  @xref{Error Recovery}.
+
+@item Token
+A basic, grammatically indivisible unit of a language.  The symbol
+that describes a token in the grammar is a terminal symbol.
+The input of the Bison parser is a stream of tokens which comes from
+the lexical analyzer.  @xref{Symbols}.
+
+@item Terminal symbol
+A grammar symbol that has no rules in the grammar and therefore is
+grammatically indivisible.  The piece of text it represents is a token.
+@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
+@end table
+
+@node Copying This Manual
+@appendix Copying This Manual
+@include fdl.texi
+
+@node Index
+@unnumbered Index
+
+@printindex cp
+
+@bye
+
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