Some code and documentation improvements.

* data/c.m4 (b4_table_value_equals): New macro to capture
some repeated code.
* data/glr.c (yyis_pact_ninf): Use it here.
(yyis_table_ninf): Likewise.
(yyreportSyntaxError): Improve internal comments.
* data/yacc.c (yyis_pact_ninf): New macro copied from glr.c.
Use it everywhere possible.
(yyis_table_ninf): Likewise.
(yysyntax_error): Improve internal comments.
* data/lalr1.cc (yysyntax_error_): Likewise.
* data/lalr1.java (yysyntax_error): Likewise.
* src/tables.h: Improve comments about yypact, yytable, etc.

data/glr.c

@@ -954,9 +954,7 @@ yylhsNonterm (yyRuleNum yyrule)
 }
 
 #define yyis_pact_ninf(yystate) \
-  ]m4_if(m4_eval(b4_pact_ninf < b4_pact_min), [1],
-	 [0],
-	 [((yystate) == YYPACT_NINF)])[
+  ]b4_table_value_equals([[pact]], [[yystate]], [b4_pact_ninf])[
 
 /** True iff LR state STATE has only a default reduction (regardless
  *  of token).  */
@@ -974,9 +972,7 @@ yydefaultAction (yyStateNum yystate)
 }
 
 #define yyis_table_ninf(yytable_value) \
-  ]m4_if(m4_eval(b4_table_ninf < b4_table_min), [1],
-	 [YYID (0)],
-	 [((yytable_value) == YYTABLE_NINF)])[
+  ]b4_table_value_equals([[table]], [[yytable_value]], [b4_table_ninf])[
 
 /** Set *YYACTION to the action to take in YYSTATE on seeing YYTOKEN.
  *  Result R means
@@ -2048,7 +2044,8 @@ yyreportSyntaxError (yyGLRStack* yystackp]b4_user_formals[)
 	  char const *yyarg[YYERROR_VERBOSE_ARGS_MAXIMUM];
 
 	  /* Start YYX at -YYN if negative to avoid negative indexes in
-	     YYCHECK.  */
+	     YYCHECK.  In other words, skip the first -YYN actions for this
+	     state because they are default actions.  */
 	  int yyxbegin = yyn < 0 ? -yyn : 0;
 
 	  /* Stay within bounds of both yycheck and yytname.  */