code: public String extractInstance(XEvent event) {
  		XAttribute attribute = event.getAttributes().get(KEY_INSTANCE);
  		if (attribute == null) {
  			return null;
  		} else {
  			return ((XAttributeLiteral) attribute).getValue();
  		}
  	}
docs: Retrieves the activity instance identifier of an event, if set by this
  extension's instance attribute.
  
  @param event
  Event to extract instance from.
  @return The requested activity instance identifier.