code: public final void addPropertyChangeListener(PropertyChangeListener l)
      {
          if (sealed) onSealedMutation();
          propertyListeners = Kit.addListener(propertyListeners, l);
      }
docs: Register an object to receive notifications when a bound property
  has changed
  @see java.beans.PropertyChangeEvent
  @see #removePropertyChangeListener(java.beans.PropertyChangeListener)
  @param l the listener