def get_activity_notification_session(self, activity_receiver=None):
raise Unimplemented()<EOL>
Gets the notification session for notifications pertaining to
        activity changes.

        arg:    activityReceiver (osid.learning.ActivityReceiver): the
                activity receiver
        return: (osid.learning.ActivityNotificationSession) - an
                ActivityNotificationSession
        raise:  NullArgument - activityReceiver is null
        raise:  OperationFailed - unable to complete request
        raise:  Unimplemented - supports_activity_notification() is
                false
        compliance: optional - This method must be implemented if
                    supports_activity_notification() is true.