setLesson(null); setPeriod(null); setRoom(null); setWrittenEvaluation(null);
removeLesson(); removeWrittenEvaluation(); removeRoom(); removePeriod();
    public void delete() {        final IPeriod period = getPeriod();        setLesson(null);        setPeriod(null);        setRoom(null);        setWrittenEvaluation(null);        period.deleteIfEmpty();        super.deleteDomainObject();    }