code: private List<Tree<K, V>> unzip(List<Tree<K, V>> zipper, boolean leftMost) {
          Tree<K, V> next = leftMost ? zipper.get(0).getLeft() : zipper.get(0).getRight();
          if (next == null)
              return zipper;
  
          return unzip(cons(next, zipper), leftMost);
      }
docs: Once a side is found to be deeper, unzip it to the bottom