docs: void doSave(XMLWriter w) throws KNXMLException
  	{
  		// <expiration timeout=int />
  		w.writeEmptyElement(TAG_EXPIRATION, Arrays
  			.asList(new Attribute[] { new Attribute(ATTR_TIMEOUT, Integer
  				.toString(timeout)) }));
  		w.writeElement(TAG_UPDATING, Collections.EMPTY_LIST, null);
  		synchronized (updating) {
  			for (final Iterator i = updating.iterator(); i.hasNext();)
  				((GroupAddress) i.next()).save(w);
  		}
  		w.endElement();
  		w.writeElement(TAG_INVALIDATING, Collections.EMPTY_LIST, null);
  		synchronized (invalidating) {
  			for (final Iterator i = invalidating.iterator(); i.hasNext();)
  				((GroupAddress) i.next()).save(w);
  		}
  		w.endElement();
  	}