def get_grade_system_notification_session(self, grade_system_receiver=None, proxy=None):
if proxy is None:<EOL><INDENT>raise NullArgument()<EOL><DEDENT>raise Unimplemented()<EOL>
Gets the notification session for notifications pertaining to grade system changes.

        arg:    grade_system_receiver
                (osid.grading.GradeSystemReceiver): the grade system
                receiver
        arg:    proxy (osid.proxy.Proxy): a proxy
        return: (osid.grading.GradeSystemNotificationSession) - a
                ``GradeSystemNotificationSession``
        raise:  NullArgument - ``grade_system_receiver`` or ``proxy`` is
                ``null``
        raise:  OperationFailed - unable to complete request
        raise:  Unimplemented - ``supports_grade_system_notification()``
                is ``false``
        *compliance: optional -- This method must be implemented if
        ``supports_grade_system_notification()`` is ``true``.*