code: def _define_format(self, occur):
           
          if hasattr(self, "default"):
              self.attr["nma:default"] = self.default
          middle = self._chorder() if self.rng_children() else "<empty/>%s"
          return (self.start_tag() + self.serialize_annots().replace("%", "%%")
                  + middle + self.end_tag())
docs: Return the serialization format for a define node.