X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/91e3ac9ab0a1fda40aeed802a158cc80a68ea960..02975b9aade52df362645b82a81ac2134290e221:/doc/bison.texinfo?ds=inline

diff --git a/doc/bison.texinfo b/doc/bison.texinfo
index 01fa9bf2..6183793b 100644
--- a/doc/bison.texinfo
+++ b/doc/bison.texinfo
@@ -191,6 +191,7 @@ Bison Grammar Files
 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.
@@ -248,7 +249,7 @@ The Lexical Analyzer Function @code{yylex}
 
 The Bison Parser Algorithm
 
-* Look-Ahead::        Parser looks one token ahead when deciding what to do.
+* 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.
@@ -311,6 +312,7 @@ Frequently Asked Questions
 * 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
@@ -330,9 +332,9 @@ Copying This Manual
 @unnumbered Introduction
 @cindex introduction
 
-@dfn{Bison} is a general-purpose parser generator that converts a grammar
-description for an @acronym{LALR}(1) or @acronym{GLR} context-free grammar
-into a C or C++ program to parse that grammar.  Once you are proficient with
+@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.
 
@@ -355,9 +357,10 @@ This edition corresponds to version @value{VERSION} of Bison.
 @node Conditions
 @unnumbered Conditions for Using Bison
 
-As of Bison version 1.24, we have changed the distribution terms for
-@code{yyparse} to permit using Bison's output in nonfree programs when
-Bison is generating C code for @acronym{LALR}(1) parsers.  Formerly, these
+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
@@ -369,10 +372,10 @@ 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
-@code{yyparse} function.  (The actions from your grammar are inserted
-into this function at one point, but the rest of the function is not
-changed.)  When we applied the @acronym{GPL} terms to the code for
-@code{yyparse},
+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
@@ -382,13 +385,11 @@ 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 only when Bison is generating C code for an
-@acronym{LALR}(1) parser; otherwise, the @acronym{GPL} terms operate
-as usual.  You can
-tell whether the exception applies to your @samp{.c} output file by
-inspecting it to see whether it says ``As a special exception, when
-this file is copied by Bison into a Bison output file, you may use
-that output file without restriction.''
+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.
 
 @include gpl.texi
 
@@ -445,7 +446,7 @@ 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 look-ahead.  Strictly speaking, that is a description of an
+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
@@ -461,11 +462,11 @@ more information on this.
 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{look-ahead}) of the remaining input.  A context-free
+(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
-look-ahead always suffices to determine the next grammar rule to apply.
+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
@@ -746,7 +747,7 @@ merged result.
 
 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 look-ahead,
+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}).
@@ -786,7 +787,7 @@ type enum = (a);
 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 look-ahead it is not
+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
@@ -825,7 +826,7 @@ 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
-look-ahead than the underlying @acronym{LALR}(1) algorithm actually allows
+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.
 
@@ -1106,13 +1107,13 @@ action in a @acronym{GLR} parser.
 @vindex yylloc
 @cindex @acronym{GLR} parsers and @code{yylloc}
 In any semantic action, you can examine @code{yychar} to determine the type of
-the look-ahead token present at the time of the associated reduction.
+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
-look-ahead token's semantic value and location, if any.
+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{Look-Ahead, ,Look-Ahead Tokens}.
+@xref{Lookahead, ,Lookahead Tokens}.
 
 @findex yyclearin
 @cindex @acronym{GLR} parsers and @code{yyclearin}
@@ -1998,7 +1999,9 @@ 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}.
+@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}
@@ -2614,6 +2617,7 @@ 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.
@@ -2633,7 +2637,7 @@ they precede the definition of @code{yyparse}.  You can use
 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 the first occurrence
+The @var{Prologue} section is terminated by the first occurrence
 of @samp{%@}} that is outside a comment, a string literal, or a
 character constant.
 
@@ -2647,6 +2651,7 @@ can be done with two @var{Prologue} blocks, one before and one after the
 
 @smallexample
 %@{
+  #define _GNU_SOURCE
   #include <stdio.h>
   #include "ptypes.h"
 %@}
@@ -2664,6 +2669,284 @@ can be done with two @var{Prologue} blocks, one before and one after the
 @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 %requires
+@findex %provides
+@findex %code-top
+The functionality of @var{Prologue} sections can often be subtle and
+inflexible.
+As an alternative, Bison provides a set of more explicit directives:
+@code{%code}, @code{%requires}, @code{%provides}, and @code{%code-top}.
+@xref{Table of Symbols,,Bison Symbols}.
+
+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 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
+@code{%code-top} and @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>
+  /* The following code really belongs in a %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 @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 need to appear in the parser code file.
+The fourth line is required by @code{YYSTYPE} and thus also needs to appear in
+the parser code file.
+However, if you've instructed Bison to generate a parser header file
+(@pxref{Table of Symbols, ,%defines}), you probably want the fourth line to
+appear before the @code{YYSTYPE} definition in that header file as well.
+Also, the @code{YYLTYPE} definition should 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 for externally exposed definitions (@code{YYSTYPE}
+and @code{YYLTYPE}) required by Bison.
+Thus, they belong in one or more @code{%requires}:
+
+@smallexample
+%code-top @{
+  #define _GNU_SOURCE
+  #include <stdio.h>
+@}
+
+%requires @{
+  #include "ptypes.h"
+@}
+%union @{
+  long int n;
+  tree t;  /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%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 code file and the parser header file.
+(By the same reasoning, @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{%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
+code file and that has no special need to appear at the top of the code file,
+you should prefer @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 @code{%code} and @code{%requires} to be
+the most important of the four @var{Prologue} alternative directives discussed
+in this section.
+
+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 code file.
+Since this function is not a dependency of any Bison-required definition (such
+as @code{YYSTYPE}), it doesn't make sense to move its prototype to a
+@code{%requires}.
+More importantly, since it depends upon @code{YYLTYPE} and @code{yytokentype},
+@code{%requires} is not sufficient.
+Instead, move its prototype from the @code{%code} to a @code{%provides}:
+
+@smallexample
+%code-top @{
+  #define _GNU_SOURCE
+  #include <stdio.h>
+@}
+
+%requires @{
+  #include "ptypes.h"
+@}
+%union @{
+  long int n;
+  tree t;  /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%requires @{
+  #define YYLTYPE YYLTYPE
+  typedef struct YYLTYPE
+  @{
+    int first_line;
+    int first_column;
+    int last_line;
+    int last_column;
+    char *filename;
+  @} YYLTYPE;
+@}
+
+%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 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 code and header files:
+@code{%code-top}, @code{%requires}, @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
+%requires @{ #include "type1.h" @}
+%union @{ type1 field1; @}
+%destructor @{ type1_free ($$); @} <field1>
+%printer @{ type1_print ($$); @} <field1>
+
+%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.
+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} alternative directives 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{%requires}, @code{%provides}, or @code{%code-top} as needed.
+
 @node Bison Declarations
 @subsection The Bison Declarations Section
 @cindex Bison declarations (introduction)
@@ -3059,7 +3342,8 @@ 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's default is to use type @code{int} for all semantic values.  To
+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
@@ -3067,6 +3351,8 @@ specify some other type, define @code{YYSTYPE} as a macro, like this:
 @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}).
 
@@ -3084,9 +3370,11 @@ requires you to do two things:
 
 @itemize @bullet
 @item
-Specify the entire collection of possible data types, with the
+Specify the entire collection of possible data types, either by using the
 @code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of
-Value Types}).
+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
@@ -3187,7 +3475,7 @@ 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 look-ahead token, if
+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}.
@@ -3310,8 +3598,8 @@ 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 for a
-mid-rule action's semantic value.
+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:
@@ -3375,8 +3663,8 @@ 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{look-ahead} token at this time, since the parser is still
-deciding what to do about it.  @xref{Look-Ahead, ,Look-Ahead Tokens}.)
+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:
@@ -3463,7 +3751,9 @@ actions to take when rules are matched.
 Defining a data type for locations is much simpler than for semantic values,
 since all tokens and groupings always use the same type.
 
-The type of locations is specified by defining a macro called @code{YYLTYPE}.
+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:
 
@@ -3477,6 +3767,9 @@ typedef struct YYLTYPE
 @} 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
@@ -3547,7 +3840,7 @@ exp:    @dots{}
 @end example
 
 @vindex yylloc
-It is also possible to access the location of the look-ahead token, if any,
+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}.
@@ -3855,6 +4148,35 @@ 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
@@ -3895,7 +4217,7 @@ 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{@@$} --- initial value and location of the look-ahead --- and the
+@code{@@$} --- initial value and location of the lookahead --- and the
 @code{%parse-param}.
 @end deffn
 
@@ -3914,7 +4236,8 @@ For instance, if your locations use a file name, you may use
 @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,
@@ -3935,26 +4258,95 @@ symbol is automatically discarded.
 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.  The additional parser parameters are also
-available (@pxref{Parser Function, , The Parser Function
-@code{yyparse}}).
+with the discarded symbol, and @code{@@$} designates its location.
+The additional parser parameters are also available (@pxref{Parser Function, ,
+The Parser Function @code{yyparse}}).
+
+When a symbol is listed among @var{symbols}, its @code{%destructor} is called a
+per-symbol @code{%destructor}.
+You may also define a per-type @code{%destructor} by listing a semantic type
+tag among @var{symbols}.
+In that case, the parser will invoke this @var{code} whenever it discards any
+grammar symbol that has that semantic type tag unless that symbol has its own
+per-symbol @code{%destructor}.
+
+Finally, you can define two different kinds of default @code{%destructor}s.
+You can place each of @code{<*>} and @code{<>} in the @var{symbols} list of
+exactly one @code{%destructor} declaration in your grammar file.
+The parser will invoke the @var{code} associated with one of these whenever it
+discards any user-defined grammar symbol that has no per-symbol and no per-type
+@code{%destructor}.
+The parser uses the @var{code} for @code{<*>} in the case of such a grammar
+symbol for which you have formally declared a semantic type tag (@code{%type}
+counts as such a declaration, but @code{$<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
 
-For instance:
+@noindent
+For example:
 
 @smallexample
-%union
-@{
-  char *string;
-@}
-%token <string> STRING
-%type  <string> string
-%destructor @{ free ($$); @} STRING string
+%union @{ char *string; @}
+%token <string> STRING1
+%token <string> STRING2
+%type  <string> string1
+%type  <string> string2
+%union @{ char character; @}
+%token <character> CHR
+%type  <character> chr
+%token TAGLESS
+
+%destructor @{ @} <character>
+%destructor @{ free ($$); @} <*>
+%destructor @{ free ($$); printf ("%d", @@$.first_line); @} STRING1 string1
+%destructor @{ printf ("Discarding tagless symbol.\n"); @} <>
 @end smallexample
 
 @noindent
-guarantees that when a @code{STRING} or a @code{string} is discarded,
-its associated memory will be freed.
+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
 
@@ -3967,7 +4359,7 @@ stacked symbols popped during the first phase of error recovery,
 @item
 incoming terminals during the second phase of error recovery,
 @item
-the current look-ahead and the entire stack (except the current
+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.
@@ -4172,10 +4564,13 @@ 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}.
 
-Unless @code{YYSTYPE} is already defined as a macro, the output header
-declares @code{YYSTYPE}.  Therefore, if you are using a @code{%union}
+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
@@ -4187,7 +4582,7 @@ Parser}.
 
 If you have also used locations, the output header declares
 @code{YYLTYPE} and @code{yylloc} using a protocol similar to that of
-@code{YYSTYPE} and @code{yylval}.  @xref{Locations, ,Tracking
+the @code{YYSTYPE} macro and @code{yylval}.  @xref{Locations, ,Tracking
 Locations}.
 
 This output file is normally essential if you wish to put the definition
@@ -4195,6 +4590,16 @@ 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 %requires
+@findex %provides
+If you have declared @code{%requires} or @code{%provides}, the output
+header also contains their code.
+@xref{Table of Symbols, ,%requires}.
+@end deffn
+
+@deffn {Directive} %defines @var{defines-file}
+Same as above, but save in the file @var{defines-file}.
 @end deffn
 
 @deffn {Directive} %destructor
@@ -4202,7 +4607,7 @@ 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}"
+@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
@@ -4215,14 +4620,14 @@ grammar does not use it, using @samp{%locations} allows for more
 accurate syntax error messages.
 @end deffn
 
-@deffn {Directive} %name-prefix="@var{prefix}"
+@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}.  For example, if you use
-@samp{%name-prefix="c_"}, the names become @code{c_parse}, @code{c_lex},
+@samp{%name-prefix "c_"}, the names become @code{c_parse}, @code{c_lex},
 and so on.  In C++ parsers, it is only the surrounding namespace which is
 named @var{prefix} instead of @samp{yy}.
 @xref{Multiple Parsers, ,Multiple Parsers in the Same Program}.
@@ -4255,7 +4660,7 @@ associate errors with the parser file, treating it an independent source
 file in its own right.
 @end deffn
 
-@deffn {Directive} %output="@var{file}"
+@deffn {Directive} %output "@var{file}"
 Specify @var{file} for the parser file.
 @end deffn
 
@@ -4305,7 +4710,7 @@ The number of parser states (@pxref{Parser States}).
 
 @deffn {Directive} %verbose
 Write an extra output file containing verbose descriptions of the
-parser states and what is done for each type of look-ahead token in
+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
@@ -4853,14 +5258,14 @@ Return immediately from @code{yyparse}, indicating success.
 @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 look-ahead token.
+a single value, and only when there is no lookahead token.
 It is also disallowed in @acronym{GLR} parsers.
-It installs a look-ahead token with token type @var{token} and
+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 look-ahead token already, then it reports a syntax error with
+a lookahead token already, then it reports a syntax error with
 a message @samp{cannot back up} and performs ordinary error
 recovery.
 
@@ -4869,12 +5274,12 @@ In either case, the rest of the action is not executed.
 
 @deffn {Macro} YYEMPTY
 @vindex YYEMPTY
-Value stored in @code{yychar} when there is no look-ahead token.
+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 look-ahead is the end of the input
+Value stored in @code{yychar} when the lookahead is the end of the input
 stream.
 @end deffn
 
@@ -4888,22 +5293,23 @@ the @samp{YYERROR;} statement.  @xref{Error Recovery}.
 @end deffn
 
 @deffn {Macro} YYRECOVERING
-This macro stands for an expression that has the value 1 when the parser
-is recovering from a syntax error, and 0 the rest of the time.
+@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 look-ahead token, or @code{YYEOF} when the
-look-ahead is the end of the input stream, or @code{YYEMPTY} when no look-ahead
+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{Look-Ahead, ,Look-Ahead Tokens}.
+@xref{Lookahead, ,Lookahead Tokens}.
 @end deffn
 
 @deffn {Macro} yyclearin;
-Discard the current look-ahead token.  This is useful primarily in
+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}).
@@ -4917,7 +5323,7 @@ errors.  This is useful primarily in error rules.
 @end deffn
 
 @deffn {Variable} yylloc
-Variable containing the look-ahead token location when @code{yychar} is not set
+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}).
@@ -4925,7 +5331,7 @@ Actions}).
 @end deffn
 
 @deffn {Variable} yylval
-Variable containing the look-ahead token semantic value when @code{yychar} is
+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}).
@@ -5106,7 +5512,7 @@ to a single grouping whose symbol is the grammar's start-symbol
 This kind of parser is known in the literature as a bottom-up parser.
 
 @menu
-* Look-Ahead::        Parser looks one token ahead when deciding what to do.
+* 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.
@@ -5117,9 +5523,9 @@ This kind of parser is known in the literature as a bottom-up parser.
 * Memory Management:: What happens when memory is exhausted.  How to avoid it.
 @end menu
 
-@node Look-Ahead
-@section Look-Ahead Tokens
-@cindex look-ahead token
+@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
@@ -5128,15 +5534,15 @@ 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{look-ahead token}, which is not on the stack.  Now the parser can
+@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 look-ahead token remains off to the side.  When no more reductions
-should take place, the look-ahead token is shifted onto the stack.  This
+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 look-ahead token, some rules may choose to delay their
+token type of the lookahead token, some rules may choose to delay their
 application.
 
-Here is a simple case where look-ahead is needed.  These three rules define
+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.
 
@@ -5171,7 +5577,7 @@ doing so would produce on the stack the sequence of symbols @code{expr
 @vindex yychar
 @vindex yylval
 @vindex yylloc
-The look-ahead token is stored in the variable @code{yychar}.
+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}.
@@ -5199,7 +5605,7 @@ if_stmt:
 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 look-ahead token, the
+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
@@ -5333,7 +5739,7 @@ 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 look-ahead token is @samp{-}: shifting
+contains @w{@samp{1 - 2}} and the lookahead token is @samp{-}: shifting
 makes right-associativity.
 
 @node Using Precedence
@@ -5394,7 +5800,7 @@ 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 look-ahead token.  If the
+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
@@ -5403,7 +5809,7 @@ precedence level.  The verbose output file made by @samp{-v}
 resolved.
 
 Not all rules and not all tokens have precedence.  If either the rule or
-the look-ahead token has no precedence, then the default is to shift.
+the lookahead token has no precedence, then the default is to shift.
 
 @node Contextual Precedence
 @section Context-Dependent Precedence
@@ -5497,9 +5903,9 @@ 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 look-ahead token is read, the current parser state together
-with the type of look-ahead token are looked up in a table.  This table
-entry can say, ``Shift the look-ahead token.''  In this case, it also
+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
@@ -5507,7 +5913,7 @@ 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 look-ahead token
+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}).
 
@@ -5665,7 +6071,7 @@ name_list:
 @end example
 
 It would seem that this grammar can be parsed with only a single token
-of look-ahead: when a @code{param_spec} is being read, an @code{ID} is
+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).
 
@@ -5679,7 +6085,7 @@ 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 look-ahead tokens in the two
+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).
@@ -5752,13 +6158,13 @@ pp.@: 615--649 @uref{http://doi.acm.org/10.1145/69622.357187}.
 
 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 look-ahead.
+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
-look-ahead, since the parser lacks the information necessary to make a
+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
@@ -5935,7 +6341,7 @@ 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
-look-ahead token is not acceptable to be shifted next, the parser reads
+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
@@ -5986,7 +6392,7 @@ error messages will be suppressed.  This macro requires no arguments;
 @samp{yyerrok;} is a valid C statement.
 
 @findex yyclearin
-The previous look-ahead token is reanalyzed immediately after an error.  If
+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.
@@ -5995,14 +6401,14 @@ action.
 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 look-ahead token ought to be discarded
+probably correct.  The previous lookahead token ought to be discarded
 with @samp{yyclearin;}.
 
 @vindex YYRECOVERING
-The macro @code{YYRECOVERING} stands for an expression that has the
-value 1 when the parser is recovering from a syntax error, and 0 the
-rest of the time.  A value of 1 indicates that error messages are
-currently suppressed for new syntax errors.
+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
@@ -6236,8 +6642,7 @@ 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 @acronym{VCG}
-file).
+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
@@ -6383,16 +6788,16 @@ 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 look-ahead is a @code{NUM}, then this token is shifted on
+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
-look-ahead triggers a syntax error.''
+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 look-ahead token because @code{NUM} can be
+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
@@ -6426,7 +6831,7 @@ state 1
 @end example
 
 @noindent
-the rule 5, @samp{exp: NUM;}, is completed.  Whatever the look-ahead token
+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}).
@@ -6449,7 +6854,7 @@ state 2
 
 @noindent
 In state 2, the automaton can only shift a symbol.  For instance,
-because of the item @samp{exp -> exp . '+' exp}, if the look-ahead if
+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
@@ -6526,7 +6931,7 @@ state 8
     $default    reduce using rule 1 (exp)
 @end example
 
-Indeed, there are two actions associated to the look-ahead @samp{/}:
+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
@@ -6544,14 +6949,14 @@ 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 look-ahead is required to select the action.  State 8 is
-one such state: if the look-ahead is @samp{*} or @samp{/} then the action
+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
-look-ahead token is @samp{*}, since we specified that @samp{*} has higher
+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 look-ahead tokens.  When run with
-@option{--report=look-ahead}, Bison specifies these look-ahead tokens:
+with some set of possible lookahead tokens.  When run with
+@option{--report=lookahead}, Bison specifies these lookahead tokens:
 
 @example
 state 8
@@ -6668,7 +7073,7 @@ The trace facility outputs messages with macro calls of the form
 @var{format} and @var{args} are the usual @code{printf} format and
 arguments.  If you define @code{YYDEBUG} to a nonzero value but do not
 define @code{YYFPRINTF}, @code{<stdio.h>} is automatically included
-and @code{YYPRINTF} is defined to @code{fprintf}.
+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}.
@@ -6825,9 +7230,12 @@ 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}.  Thus, the following shell script can substitute
-for Yacc, and the Bison distribution contains such a script for
-compatibility with @acronym{POSIX}:
+@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
@@ -6861,7 +7269,7 @@ 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.
+Pretend that @code{%name-prefix "@var{prefix}"} was specified.
 @xref{Decl Summary}.
 
 @item -l
@@ -6896,7 +7304,7 @@ Same as above, but save in the file @var{defines-file}.
 
 @item -b @var{file-prefix}
 @itemx --file-prefix=@var{prefix}
-Pretend that @code{%file-prefix} was specified, i.e, specify prefix to use
+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}
@@ -6909,9 +7317,9 @@ separated list of @var{things} among:
 Description of the grammar, conflicts (resolved and unresolved), and
 @acronym{LALR} automaton.
 
-@item look-ahead
+@item lookahead
 Implies @code{state} and augments the description of the automaton with
-each rule's look-ahead set.
+each rule's lookahead set.
 
 @item itemset
 Implies @code{state} and augments the description of the automaton with
@@ -6920,7 +7328,7 @@ the full set of items for each state, instead of its core only.
 
 @item -v
 @itemx --verbose
-Pretend that @code{%verbose} was specified, i.e, write an extra output
+Pretend that @code{%verbose} was specified, i.e., write an extra output
 file containing verbose descriptions of the grammar and
 parser.  @xref{Decl Summary}.
 
@@ -6932,10 +7340,11 @@ The other output files' names are constructed from @var{file} as
 described under the @samp{-v} and @samp{-d} options.
 
 @item -g
-Output a @acronym{VCG} definition of the @acronym{LALR}(1) grammar
-automaton computed by Bison.  If the grammar file is @file{foo.y}, the
-@acronym{VCG} output file will
-be @file{foo.vcg}.
+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.
+If the grammar file is @file{foo.y}, the output file will
+be @file{foo.dot}.
 
 @item --graph=@var{graph-file}
 The behavior of @var{--graph} is the same than @samp{-g}.  The only
@@ -7040,13 +7449,13 @@ An auxiliary class @code{stack} used by the parser.
 
 @item @var{file}.hh
 @itemx @var{file}.cc
-The declaration and implementation of the C++ parser class.
-@var{file} is the name of the output file.  It follows the same
-rules as with regular C parsers.
+(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}).
 
-Note that @file{@var{file}.hh} is @emph{mandatory}, the C++ cannot
-work without the parser class declaration.  Therefore, you must either
-pass @option{-d}/@option{--defines} to @command{bison}, or use the
+The header is @emph{mandatory}; you must either pass
+@option{-d}/@option{--defines} to @command{bison}, or use the
 @samp{%defines} directive.
 @end table
 
@@ -7290,8 +7699,8 @@ factor both as follows.
 
 @comment file: calc++-driver.hh
 @example
-// Announce to Flex the prototype we want for lexing function, ...
-# define YY_DECL					\
+// 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,      \
@@ -7321,7 +7730,6 @@ public:
 @noindent
 To encapsulate the coordination with the Flex scanner, it is useful to
 have two members function to open and close the scanning phase.
-members.
 
 @comment file: calc++-driver.hh
 @example
@@ -7420,19 +7828,21 @@ the grammar for.
 @end example
 
 @noindent
+@findex %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{Table of Symbols, ,%requires}.
 
 @comment file: calc++-parser.yy
 @example
-%@{
+%requires @{
 # include <string>
 class calcxx_driver;
-%@}
+@}
 @end example
 
 @noindent
@@ -7488,15 +7898,15 @@ them.
 @end example
 
 @noindent
-The code between @samp{%@{} and @samp{%@}} after the introduction of the
-@samp{%union} is output in the @file{*.cc} file; it needs detailed
-knowledge about the driver.
+@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
 
 
@@ -7541,7 +7951,9 @@ unit: assignments exp  @{ driver.result = $2; @};
 assignments: assignments assignment @{@}
            | /* Nothing.  */        @{@};
 
-assignment: "identifier" ":=" exp @{ driver.variables[*$1] = $3; @};
+assignment:
+     "identifier" ":=" exp
+       @{ driver.variables[*$1] = $3; delete $1; @};
 
 %left '+' '-';
 %left '*' '/';
@@ -7549,7 +7961,7 @@ exp: exp '+' exp   @{ $$ = $1 + $3; @}
    | exp '-' exp   @{ $$ = $1 - $3; @}
    | exp '*' exp   @{ $$ = $1 * $3; @}
    | exp '/' exp   @{ $$ = $1 / $3; @}
-   | "identifier"  @{ $$ = driver.variables[*$1]; @}
+   | "identifier"  @{ $$ = driver.variables[*$1]; delete $1; @}
    | "number"      @{ $$ = $1; @};
 %%
 @end example
@@ -7583,10 +7995,14 @@ parser's to get the set of defined tokens.
 # include <string>
 # include "calc++-driver.hh"
 # include "calc++-parser.hh"
-/* Work around a bug in flex 2.5.31.  See Debian bug 333231
-  <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=333231>.  */
+
+/* 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.  */
@@ -7706,8 +8122,8 @@ main (int argc, char *argv[])
       driver.trace_scanning = true;
     else
       @{
-	driver.parse (*argv);
-	std::cout << driver.result << std::endl;
+        driver.parse (*argv);
+        std::cout << driver.result << std::endl;
       @}
 @}
 @end example
@@ -7727,6 +8143,7 @@ are addressed.
 * 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
@@ -7927,6 +8344,55 @@ 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?
 
@@ -8104,12 +8570,76 @@ 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}.
+@xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
+
+@deffn {Directive} <>
+Used to define a default tagless @code{%destructor} or default tagless
+@code{%printer}.
+@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}@}
+Other than semantic actions, this is probably the most common place you should
+write verbatim code for the parser implementation.
+For C/C++, it replaces the traditional Yacc prologue,
+@code{%@{@var{code}%@}}, for most purposes.
+For Java, it inserts code into the parser class.
+
+@cindex Prologue
+@findex %union
+Compare with @code{%@{@var{code}%@}} (@pxref{Prologue, ,The Prologue})
+appearing after the first @code{%union @{@var{code}@}} in a C/C++ based grammar
+file.
+While Bison will continue to support @code{%@{@var{code}%@}} for backward
+compatibility, @code{%code @{@var{code}@}} is cleaner as its functionality does
+not depend on its position in the grammar file relative to any
+@code{%union @{@var{code}@}}.
+Specifically, @code{%code @{@var{code}@}} always inserts your @var{code} into
+the parser code file after the usual contents of the parser header file.
+
+@xref{Prologue Alternatives}.
+@end deffn
+
+@deffn {Directive} %code-top @{@var{code}@}
+Occasionally for C/C++ it is desirable to insert code near the top of the
+parser code file.
+For example:
+
+@smallexample
+%code-top @{
+  #define _GNU_SOURCE
+  #include <stdio.h>
+@}
+@end smallexample
+
+@noindent
+For Java, @code{%code-top @{@var{code}@}} is currently unused.
+
+@cindex Prologue
+@findex %union
+Compare with @code{%@{@var{code}%@}} appearing before the first
+@code{%union @{@var{code}@}} in a C/C++ based grammar file.
+@code{%code-top @{@var{code}@}} is cleaner as its functionality does not depend
+on its position in the grammar file relative to any
+@code{%union @{@var{code}@}}.
+
+@xref{Prologue Alternatives}.
+@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
@@ -8127,6 +8657,11 @@ 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}.
@@ -8148,8 +8683,8 @@ 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 look-ahead token.  Actions
-corresponding to @code{error} are then executed, and the look-ahead
+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
@@ -8159,7 +8694,7 @@ Bison declaration to request verbose, specific error message strings
 when @code{yyerror} is called.
 @end deffn
 
-@deffn {Directive} %file-prefix="@var{prefix}"
+@deffn {Directive} %file-prefix "@var{prefix}"
 Bison declaration to set the prefix of the output files.  @xref{Decl
 Summary}.
 @end deffn
@@ -8191,7 +8726,7 @@ 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}"
+@deffn {Directive} %name-prefix "@var{prefix}"
 Bison declaration to rename the external symbols.  @xref{Decl Summary}.
 @end deffn
 
@@ -8213,7 +8748,7 @@ Bison declaration to assign nonassociativity to token(s).
 @xref{Precedence Decl, ,Operator Precedence}.
 @end deffn
 
-@deffn {Directive} %output="@var{file}"
+@deffn {Directive} %output "@var{file}"
 Bison declaration to set the name of the parser file.  @xref{Decl
 Summary}.
 @end deffn
@@ -8229,6 +8764,18 @@ Bison declaration to assign a precedence to a specific rule.
 @xref{Contextual Precedence, ,Context-Dependent Precedence}.
 @end deffn
 
+@deffn {Directive} %provides @{@var{code}@}
+This is the right place to write additional definitions you would like Bison to
+expose externally.
+For C/C++, this directive inserts your @var{code} both into the parser header
+file (if generated; @pxref{Table of Symbols, ,%defines}) and into the parser
+code file after Bison's required definitions.
+For Java, it inserts your @var{code} into the parser java file after the parser
+class.
+
+@xref{Prologue Alternatives}.
+@end deffn
+
 @deffn {Directive} %pure-parser
 Bison declaration to request a pure (reentrant) parser.
 @xref{Pure Decl, ,A Pure (Reentrant) Parser}.
@@ -8239,6 +8786,29 @@ Require version @var{version} or higher of Bison.  @xref{Require Decl, ,
 Require a Version of Bison}.
 @end deffn
 
+@deffn {Directive} %requires @{@var{code}@}
+This is the right place to write dependency code for externally exposed
+definitions required by Bison.
+For C/C++, such exposed definitions are those usually appearing in the parser
+header file.
+Thus, this is the right place to define types referenced in
+@code{%union @{@var{code}@}} directives, and it is the right place to override
+Bison's default @code{YYSTYPE} and @code{YYLTYPE} definitions.
+For Java, this is the right place to write import directives.
+
+@cindex Prologue
+@findex %union
+Compare with @code{%@{@var{code}%@}} (@pxref{Prologue, ,The Prologue})
+appearing before the first @code{%union @{@var{code}@}} in a C/C++ based
+grammar file.
+Unlike @code{%@{@var{code}%@}}, @code{%requires @{@var{code}@}} inserts your
+@var{code} both into the parser code file and into the parser header file (if
+generated; @pxref{Table of Symbols, ,%defines}) since Bison's required
+definitions should depend on it in both places.
+
+@xref{Prologue Alternatives}.
+@end deffn
+
 @deffn {Directive} %right
 Bison declaration to assign right associativity to token(s).
 @xref{Precedence Decl, ,Operator Precedence}.
@@ -8289,20 +8859,20 @@ read, by making @code{yyparse} return 0 immediately.
 @end deffn
 
 @deffn {Macro} YYBACKUP
-Macro to discard a value from the parser stack and fake a look-ahead
+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
-look-ahead token.  (In a pure parser, it is a local variable within
+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
-look-ahead token.  @xref{Error Recovery}.
+lookahead token.  @xref{Error Recovery}.
 @end deffn
 
 @deffn {Macro} YYDEBUG
@@ -8368,7 +8938,7 @@ variable within @code{yyparse}, and its address is passed to
 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 look-ahead token.
+In semantic actions, it stores the location of the lookahead token.
 @xref{Actions and Locations, ,Actions and Locations}.
 @end deffn
 
@@ -8383,7 +8953,7 @@ 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 look-ahead token.
+In semantic actions, it stores the semantic value of the lookahead token.
 @xref{Actions, ,Actions}.
 @end deffn
 
@@ -8411,8 +8981,9 @@ Conventions for Pure Parsers}.
 @end deffn
 
 @deffn {Macro} YYRECOVERING
-Macro whose value indicates whether the parser is recovering from a
-syntax error.  @xref{Action Features, ,Special Features for Use in Actions}.
+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
@@ -8522,8 +9093,8 @@ tokens are parsed.  @xref{Lexical Tie-ins}.
 @item Literal string token
 A token which consists of two or more fixed characters.  @xref{Symbols}.
 
-@item Look-ahead token
-A token already read but not yet shifted.  @xref{Look-Ahead, ,Look-Ahead
+@item Lookahead token
+A token already read but not yet shifted.  @xref{Lookahead, ,Lookahead
 Tokens}.
 
 @item @acronym{LALR}(1)
@@ -8533,7 +9104,7 @@ Conflicts, ,Mysterious Reduce/Reduce Conflicts}.
 
 @item @acronym{LR}(1)
 The class of context-free grammars in which at most one token of
-look-ahead is needed to disambiguate the parsing of any piece of input.
+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
@@ -8647,9 +9218,9 @@ grammatically indivisible.  The piece of text it represents is a token.
 @c LocalWords: strncmp intval tindex lvalp locp llocp typealt YYBACKUP
 @c LocalWords: YYEMPTY YYEOF YYRECOVERING yyclearin GE def UMINUS maybeword
 @c LocalWords: Johnstone Shamsa Sadaf Hussain Tomita TR uref YYMAXDEPTH
-@c LocalWords: YYINITDEPTH stmnts ref stmnt initdcl maybeasm VCG notype
+@c LocalWords: YYINITDEPTH stmnts ref stmnt initdcl maybeasm notype
 @c LocalWords: hexflag STR exdent itemset asis DYYDEBUG YYFPRINTF args
-@c LocalWords: YYPRINTF infile ypp yxx outfile itemx vcg tex leaderfill
+@c LocalWords: infile ypp yxx outfile itemx tex leaderfill
 @c LocalWords: hbox hss hfill tt ly yyin fopen fclose ofirst gcc ll
 @c LocalWords: yyrestart nbar yytext fst snd osplit ntwo strdup AST
 @c LocalWords: YYSTACK DVI fdl printindex