if (l == null) return;
  public synchronized void addPropertyChangeListener(PropertyChangeListener l)  {    if (l instanceof PropertyChangeListenerProxy)      {        PropertyChangeListenerProxy p = (PropertyChangeListenerProxy) l;        addPropertyChangeListener(p.propertyName,                                  (PropertyChangeListener) p.getListener());      }    else      {        if (listeners == null)          listeners = new Vector();        listeners.add(l);      }  }