@abc.abstractmethod<EOL><INDENT>def get_relationship_ids_by_families(self, family_ids):<DEDENT>
return<EOL>
Gets the list of ``Relationship Ids`` corresponding to a list of ``Family`` objects.

        :param family_ids: list of family ``Ids``
        :type family_ids: ``osid.id.IdList``
        :return: list of relationship ``Ids``
        :rtype: ``osid.id.IdList``
        :raise: ``NullArgument`` -- ``family_ids`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``PermissionDenied`` -- authorization failure

        *compliance: mandatory -- This method must be implemented.*