def rollback_service(self, rollback_time=None):
<EOL>raise Unimplemented()<EOL>
Rolls back this service to a point in time.

        arg:    rollbackTime (timestamp): the requested time
        return: (osid.journaling.JournalEntry) - the journal entry
                corresponding to the actual state of this service
        raise:  OperationFailed - unable to complete request
        raise:  PERMISSION_DENIED - authorization failure occurred
        raise:  Unimplemented - supports_journal_rollback() is false
        compliance: mandatory - This method must be implemented.