PG generates wrong grammar rules for optional element

use case:

args_OPT  
   :      {_term("args_OPT");}    MT_args_OPT  
   |     {_embed();}    ET_args_OPT      CRSX_EMBED_END  
   |     {}  {_enterOPT();}    args     ?     ({_tail();} ({_term("args_OPT");}  MT_args_OPT  | {_embed();} ET_args_OPT      CRSX_EMBED_END))? {_exitOPT();}  {}  ;

Tail matching should not be allowed.
