code: @Override
  	public Comparator<? super T> comparator() {
  		return new Comparator<T>() {
  			@Override
  			public int compare(T o1, T o2) {
  				// compare elements according to the universe ordering
  				return itemToIndex.get(o1).compareTo(itemToIndex.get(o2));
  			}
  		};
  	}
docs: {@inheritDoc}