m_causes.remove(ev);
    private void clearArrows(Event ev) {        Collection outgoing = getOutgoing(ev);        if (outgoing == null) { return; }        for (Iterator it = outgoing.iterator(); it.hasNext(); ) {            Event to = (Event) it.next();            setCount(to, getCount(to) - 1);        }        m_outgoing.remove(ev);    }