setPreviousPeriod(null); setNextPeriod(null);
removeNextPeriod(); removePreviousPeriod();
    private void delete() {        final IPeriod previous = getPreviousPeriod();        final IPeriod next = getNextPeriod();        if (previous != null && next != null) {             previous.setNextPeriod(next);        } else {            setPreviousPeriod(null);            setNextPeriod(null);        }        deleteDomainObject();    }