code: public void registerListener(ResponseListener listener) {
  		if (! listeners.contains(listener)) {
  			boolean unique = true;
  			
  			for (ResponseListener existing : listeners) {
  				if (existing.getClass().equals(listener.getClass())) {
  					unique = false;
  					break;
  				}
  			}
  			
  			if (unique) {
  				listeners.add(listener);
  			}
  		}
  	}
docs: Register an {@link ResponseListener} to notify when {@link Response}s are added
  
  @param listener the {@link ResponseListener} to register