@abc.abstractmethod<EOL><INDENT>def get_assignable_gradebook_ids(self, gradebook_id):<DEDENT>
return<EOL>
Gets a list of gradebook ``Ids`` including and under the given gradebook node in which any gradebook column can be assigned.

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

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