@abc.abstractmethod<EOL><INDENT>def get_gradebook_column_notification_session(self, gradebook_column_receiver, proxy):<DEDENT>
return<EOL>
Gets the notification session for notifications pertaining to gradebook column changes.

        :param gradebook_column_receiver: the grade system receiver
        :type gradebook_column_receiver: ``osid.grading.GradebookColumnReceiver``
        :param proxy: a proxy
        :type proxy: ``osid.proxy.Proxy``
        :return: a ``GradebookColumnNotificationSession``
        :rtype: ``osid.grading.GradebookColumnNotificationSession``
        :raise: ``NullArgument`` -- ``gradebook_column_receiver`` or ``proxy`` is ``null``
        :raise: ``OperationFailed`` -- unable to complete request
        :raise: ``Unimplemented`` -- ``supports_gradebook_column_notification()`` is ``false``

        *compliance: optional -- This method must be implemented if
        ``supports_gradebook_column_notification()`` is ``true``.*