code: public void addPropertyChangeListener(String propertyName, PropertyChangeListener listener) {
  		if (children == null) {
  			children = new HashMap();
  		}
  		PropertyChangeSupport child = (PropertyChangeSupport) children.get(propertyName);
  		if (child == null) {
  			child = new PropertyChangeSupport(source);
  			children.put(propertyName, child);
  		}
  		child.addPropertyChangeListener(listener);
  	}
docs: Add a PropertyChangeListener for a specific property. The listener will
  be invoked only when a call on firePropertyChange names that specific
  property.
  
  @param propertyName The name of the property to listen on.
  @param listener The PropertyChangeListener to be added