Serialize the value directory on the parent.

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)