code: @SuppressWarnings("unchecked")
      private Zipper<K, V> compareDepth(Tree<K, V> left, Tree<K, V> right) {
          return unzipBoth(left, right, (List<Tree<K, V>>) Collections.EMPTY_LIST, (List<Tree<K, V>>) Collections.EMPTY_LIST, 0);
      }
docs: If the trees were balanced, returns an empty zipper