code: def custom_str(self, sc_expr_str_fn):
          
          return self._menu_comment_node_str(sc_expr_str_fn) \
                 if self.item in _MENU_COMMENT else \
                 self._sym_choice_node_str(sc_expr_str_fn)
docs: Works like MenuNode.__str__(), but allows a custom format to be used
          for all symbol/choice references. See expr_str().