docs: def serialize_on_parent(
              self,
              parent,  # type: ET.Element
              value,  # type: Any
              state  # type: _ProcessorState
      ):
          # type: (...) -> None
          """Serialize the value directory on the parent."""
          xml_value = _hooks_apply_before_serialize(self._hooks, state, value)
          self._processor.serialize_on_parent(parent, xml_value, state)