code: public E union(E e1, E e2) {
  	Node<E> node1 = _get_or_create_root_set(e1);
  	Node<E> node2 = _get_or_create_root_set(e2);
  	return _link(node1, node2).element;
      }
docs: Unifies the elements <code>e1</code> and <code>e2</code> and
  returns the representative of the resulting equivalence class.