@abc.abstractmethod<EOL><INDENT>def get_assignable_calendar_ids(self, calendar_id):<DEDENT>
return<EOL>
Gets a list of calendars including and under the given calendar node in which any time period can be
        assigned.

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

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