Instead of mapping the LHS of unused rules to -1, keep the LHS

valid, but flag the rules as invalid.
* src/gram.h (rule_t): `useful' is a new member.
* src/print.c (print_grammar): Adjust.
* src/derives.c (set_derives): Likewise.
* src/reader.c (packgram, reduce_output): Likewise.
* src/reduce.c (reduce_grammar_tables): Likewise.
* tests/reduce.at (Underivable Rules, Useless Rules): New.

src/gram.h

@@ -59,6 +59,8 @@
 
    RULE_TABLE[R].line -- the line where R was defined.
 
+   RULE_TABLE[R].useful -- TRUE iff the rule is used.
+
    The right hand side is stored as symbol numbers in a portion of
    RITEM.
 
@@ -119,6 +121,7 @@ typedef struct rule_s
   short precsym;
   short assoc;
   short line;
+  bool useful;
 } rule_t;
 
 extern struct rule_s *rule_table;