The Prolog fact below reports detection of an Observer occurrence in JHotDraw 5.1. It tells us that:

• The Subject role is played by the type StandardDrawing.
• The observers role¹⁾ is played by the field fListeners.
• The Notifier Method role²⁾ is played by two methods: figureInvalidated and figureRequestUpdate.
• The updateCall role ³⁾ is played by the method calls l.drawingInvalidated(…) and l.drawingRequestUpdate(…)
• The Observer role is played by the type DrawingChangeListener.
• The Update Method role is played by two methods of DrawingChangeListener: drawingInvalidated(DrawingChangeEvent) and drawingRequestUpdate(DrawingChangeEvent)

candidate(observer, 100, [
  role(subject, ['CH.ifa.draw.standard.StandardDrawing']), 
  role(observers, ['CH.ifa.draw.standard.StandardDrawing.fListeners']), 
  role(notifier,[ 
   'CH.ifa.draw.standard.StandardDrawing.figureInvalidated(FigureChangeEvent)', 
   'CH.ifa.draw.standard.StandardDrawing.figureRequestUpdate(FigureChangeEvent)']
  ), 
  role(updateCall, [
   'l.drawingInvalidated(new CH.ifa.draw.framework.DrawingChangeEvent(...))',
   'l.drawingRequestUpdate(new CH.ifa.draw.framework.DrawingChangeEvent(...))'
  ]), 
  role(observer, [
   'CH.ifa.draw.framework.DrawingChangeListener']), 
  role(updatemethod, [  
   'CH.ifa.draw.framework.DrawingChangeListener.drawingInvalidated(DrawingChangeEvent)', 
   'CH.ifa.draw.framework.DrawingChangeListener.drawingRequestUpdate(DrawingChangeEvent)']
  )]
).

The above output format does not make explicit which Notifier calls which UpdateCall to which Update Method. A more explicit output format (such as DPDX) will be implemented.