-/* set_derives finds, for each variable (nonterminal), which rules can
- derive it. It sets up the value of derives so that derives[i -
- ntokens] points to a vector of rule numbers, terminated with -1.
- */
+/* DERIVES[SYMBOL - NTOKENS] points to a vector of the rules that
+ SYMBOL derives, terminated with NULL. */
+extern rule_t ***derives;