code: public boolean addWebcamListener(WebcamListener l) {
  		if (l == null) {
  			throw new IllegalArgumentException("Webcam listener cannot be null!");
  		}
  		assert listeners != null;
  		return listeners.add(l);
  	}
docs: Add webcam listener.
  
  @param l the listener to be added
  @return True if listener has been added, false if it was already there
  @throws IllegalArgumentException when argument is null