  bool walk(Item_processor processor, bool walk_subquery, void *args)
  {
    return arg->walk(processor, walk_subquery, args) ||
	    (this->*processor)(args);
  }
  bool walk(Item_processor processor, bool walk_subquery, void *args)
  {
    return arg->walk(processor, walk_subquery, args) ||
	    (this->*processor)(args);
  }