public void addProcessInteractionListener(final ProcessInteractionListener l){
  if (l == null) {
    throw new IllegalArgumentException("l must not be null!");
  }
  processInteractionListeners.add(l);
}

public void addProcessInteractionListener ( final ProcessInteractionListener l ) { if ( l == null ) { throw new IllegalArgumentException ( "l must not be null!" ) ; } processInteractionListeners . add ( l ) ; }
adds a listener that will be informed when the user right - clicks an operator or a port .
