@abc.abstractmethod<EOL><INDENT>def get_assignable_family_ids(self, family_id):<DEDENT>
return<EOL>
Gets a list of families including and under the given family node in which any relationship can be assigned.

        :param family_id: the ``Id`` of the ``Family``
        :type family_id: ``osid.id.Id``
        :return: list of assignable family ``Ids``
        :rtype: ``osid.id.IdList``
        :raise: ``NullArgument`` -- ``family_id`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request

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