@Override
 public void removeVariable(StateVariable variable, Object sourceObject) {
   synchronized (_listeners) {
     for (StateVariableListener listener : _listeners) {
       listener.removeVariable(this, sourceObject);
     }
   }
 }
 @Override
 public void notifValueChange(
     StateVariable variable, Object oldValue, Object newValue, Object sourceObject) {
   synchronized (_listeners) {
     for (StateVariableListener listener : _listeners) {
       listener.notifValueChange(this, oldValue, newValue, sourceObject);
     }
   }
 }