<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def ordered_members_map(self):
 """ Mask to group the persons by entity This function only caches the map value, to see what the map is used for, see value_nth_person method. """

        if self._ordered_members_map is None:
            return np.argsort(self.members_entity_id)
        return self._ordered_members_map