def get_assignable_family_ids(self, family_id=None):
pass<EOL>
Gets a list of families including and under the given family node in which any
            relationship can be assigned.

        arg:    family_id (osid.id.Id): the ``Id`` of the ``Family``
        return: (osid.id.IdList) - list of assignable family ``Ids``
        raise:  NullArgument - ``family_id`` is ``null``
        raise:  OperationFailed - unable to complete request
        *compliance: mandatory -- This method must be implemented.*