* Allow closures for lexing.

* Speed-up for pre-computing lexables.  Grammar::LEXABLES, etc.

* Test lexing suffixes.

* Allow macros:
     @VALUE
     $VALUE_REF
     $VALUE[n]
     $VALUE{NAME}
     $VALUE{NAME}[n]
     @RULE
     $RULE_REF

* Add locations for earlemes.  Also for rules?  Use in Carp messages.

* Distinguish proper separator rules?

* Allow alternation.

* (Someday) precedence declarations, a la yacc.

* (Later) Look at Yacc macros yyerrok YYERROR YYACCEPT YYABORT YYRECOVERING.

* (Later) Look at error recovery.   Is anything in Parse::Yapp or yacc useful?
