* src/gram.h, src/gram.c (rules_rhs_length): New.

(ritem_longest_rhs): Use it.
* src/gram.h (rule_t): `number' is a new member.
* src/reader.c (packgram): Set it.
* src/reduce.c (reduce_grammar_tables): Move the useless rules at
the end of `rules', and count them out of `nrules'.
(reduce_output, dump_grammar): Adjust.
* src/print.c (print_grammar): It is no longer needed to check for
the usefulness of a rule, as useless rules are beyond `nrules + 1'.
* tests/reduce.at (Reduced Automaton): New test.

src/gram.h

@@ -124,6 +124,10 @@ typedef enum
 
 typedef struct rule_s
 {
+  /* The number of the rule in the source.  It is usually the index in
+     RULES too, except if there are useless rules.  */
+  short number;
+
   short lhs;
   short *rhs;
   short prec;
@@ -166,6 +170,8 @@ extern int pure_parser;
 
 extern int error_token_number;
 
+/* Report the length of the RHS. */
+int rule_rhs_length PARAMS ((rule_t *rule));
 
 /* Dump RITEM for traces. */
 void ritem_print PARAMS ((FILE *out));