throw new UnsupportedOperationException();
Object key = check(o); Element child = (Element) o; if (!this.equals(child.getParent())) { throw new IllegalArgumentException ("child does not belong to this parent"); } m_children.remove(o); m_childrenMap.remove(key); child.setParent(null); return true;
    public boolean remove(Object o) {        throw new UnsupportedOperationException();    }