Serialize the value to a new element and return the element.

def serialize(
            self,
            value,  # type: Any
            state  # type: _ProcessorState
    ):
        # type: (...) -> ET.Element
        """Serialize the value to a new element and return the element."""
        if not value and self.required:
            state.raise_error(
                MissingValue, 'Missing required aggregate "{}"'.format(self.element_path)
            )

        start_element, end_element = _element_path_create_new(self.element_path)
        self._serialize(end_element, value, state)
        return start_element