def get_relationship_ids_by_families(self, family_ids=None):
pass<EOL>
Gets the list of ``Relationship Ids`` corresponding to a list of ``Family`` objects.

        arg:    family_ids (osid.id.IdList): list of family ``Ids``
        return: (osid.id.IdList) - list of relationship ``Ids``
        raise:  NullArgument - ``family_ids`` is ``null``
        raise:  OperationFailed - unable to complete request
        raise:  PermissionDenied - authorization failure
        *compliance: mandatory -- This method must be implemented.*