@Override
 public synchronized void resumeRecording() {
   Stack<Computation> current = EclipseContext.getCalculatedComputations();
   Computation plug = current.pop();
   if (plug != null)
     throw new IllegalArgumentException(
         "Internal error in nested computation processing"); //$NON-NLS-1$
 }
 @Override
 public synchronized void pauseRecording() {
   Stack<Computation> current = EclipseContext.getCalculatedComputations();
   current.push(null);
 }