code: private void checkLabels(boolean forGenerative, String... label)
  	{
  		for (int i = 0; i < (forGenerative ? label.length - 1 : label.length); i++)
  		{
  			if (!hasLabel(label[i])) throw new IllegalArgumentException(
  				"Label neither found, nor generated: " + label[i]);
  		}
  	}
docs: Checks if all labels (other than the last if generative) exists.
  @param forGenerative whether the check is performed for a generative constraint
  @param label labels to check
  @throws IllegalArgumentException when a necessary label is not found