+Changes in version 2.3+:
+
+* Except for LALR(1) parsers in C with POSIX Yacc emulation enabled (with `-y',
+ `--yacc', or `%yacc'), Bison no longer generates #define statements for
+ associating token numbers with token names. Removing the #define statements
+ helps to sanitize the global namespace during preprocessing, but POSIX Yacc
+ requires them. Bison still generates an enum for token names in all cases.
+
+* Handling of prologue blocks is now more consistent but potentially backward
+ incompatible.
+
+ As before, you declare prologue blocks in your grammar file with the
+ `%{ ... %}' syntax. To generate the pre-prologue, Bison concatenates all
+ prologue blocks that you've declared before any %union. If you've declared a
+ %union, Bison concatenates all prologue blocks that you've declared after it
+ to generate the post-prologue. (The new %before-definitions and
+ %after-definitions have a similar effect as %union on the prologues. See
+ below.)
+
+ Previous versions of Bison inserted the pre-prologue into both the header
+ file and the code file in all cases except for LALR(1) parsers in C. In the
+ latter case, Bison inserted it only into the code file. For parsers in C++,
+ the point of insertion was before any token definitions (which associate
+ token numbers with names). For parsers in C, the point of insertion was
+ after the token definitions.
+
+ Now, Bison never inserts the pre-prologue into the header file. In the code
+ file, it always inserts it before the token definitions.
+
+* Bison now provides the %before-definitions and %after-definitions directives.
+
+ For example, in your grammar file:
+
+ %{
+ /* A pre-prologue block. For Yacc portability, Bison no longer puts this
+ * in the header file. In the code file, Bison inserts it before any
+ * %before-definitions blocks. */
+ %}
+ %before-definitions {
+ /* Bison inserts this into both the header file and code file. In both
+ * files, the point of insertion is before any Bison-generated token,
+ * semantic type, location type, and class definitions. This is a good
+ * place to define %union dependencies, for example. */
+ }
+ %union {
+ /* With previous versions of Bison, the first %union in your grammar file
+ * separated the pre-prologue blocks from the post-prologue blocks. Now,
+ * the first %union, %before-definitions, or %after-definitions does
+ * that. */
+ }
+ %after-definitions {
+ /* If you want something in the header file and in the code file and it
+ * depends on any of the Bison-generated definitions in the header file,
+ * put it here. */
+ }
+ %{
+ /* A post-prologue block. If you want something in the code file but not
+ * in the header file and it depends on Bison-generated definitions, put
+ * it here. In the code file, Bison inserts it after any
+ * %after-definitions blocks. */
+ %}
+
+* The option `--report=look-ahead' has been changed to `--report=lookahead'.
+ The old spelling still works, but is not documented and may be removed
+ in a future release.
+
+Changes in version 2.3, 2006-06-05:
+
+* GLR grammars should now use `YYRECOVERING ()' instead of `YYRECOVERING',
+ for compatibility with LALR(1) grammars.
+
+* It is now documented that any definition of YYSTYPE or YYLTYPE should
+ be to a type name that does not contain parentheses or brackets.
+
+Changes in version 2.2, 2006-05-19:
+
+* The distribution terms for all Bison-generated parsers now permit
+ using the parsers in nonfree programs. Previously, this permission
+ was granted only for Bison-generated LALR(1) parsers in C.
+
+* %name-prefix changes the namespace name in C++ outputs.
+
+* The C++ parsers export their token_type.
+
+* Bison now allows multiple %union declarations, and concatenates
+ their contents together.
+
+* New warning: unused values
+ Right-hand side symbols whose values are not used are reported,
+ if the symbols have destructors. For instance:
+
+ exp: exp "?" exp ":" exp { $1 ? $1 : $3; }
+ | exp "+" exp
+ ;
+
+ will trigger a warning about $$ and $5 in the first rule, and $3 in
+ the second ($1 is copied to $$ by the default rule). This example
+ most likely contains three errors, and could be rewritten as:
+
+ exp: exp "?" exp ":" exp
+ { $$ = $1 ? $3 : $5; free ($1 ? $5 : $3); free ($1); }
+ | exp "+" exp
+ { $$ = $1 ? $1 : $3; if ($1) free ($3); }
+ ;
+
+ However, if the original actions were really intended, memory leaks
+ and all, the warnings can be suppressed by letting Bison believe the
+ values are used, e.g.:
+
+ exp: exp "?" exp ":" exp { $1 ? $1 : $3; (void) ($$, $5); }
+ | exp "+" exp { $$ = $1; (void) $3; }
+ ;
+
+ If there are mid-rule actions, the warning is issued if no action
+ uses it. The following triggers no warning: $1 and $3 are used.
+
+ exp: exp { push ($1); } '+' exp { push ($3); sum (); };
+
+ The warning is intended to help catching lost values and memory leaks.
+ If a value is ignored, its associated memory typically is not reclaimed.
+
+* %destructor vs. YYABORT, YYACCEPT, and YYERROR.
+ Destructors are now called when user code invokes YYABORT, YYACCEPT,
+ and YYERROR, for all objects on the stack, other than objects
+ corresponding to the right-hand side of the current rule.
+
+* %expect, %expect-rr
+ Incorrect numbers of expected conflicts are now actual errors,
+ instead of warnings.
+
+* GLR, YACC parsers.
+ The %parse-params are available in the destructors (and the
+ experimental printers) as per the documentation.
+
+* Bison now warns if it finds a stray `$' or `@' in an action.
+
+* %require "VERSION"
+ This specifies that the grammar file depends on features implemented
+ in Bison version VERSION or higher.
+
+* lalr1.cc: The token and value types are now class members.
+ The tokens were defined as free form enums and cpp macros. YYSTYPE
+ was defined as a free form union. They are now class members:
+ tokens are enumerations of the `yy::parser::token' struct, and the
+ semantic values have the `yy::parser::semantic_type' type.
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