checkNotNull(root);
    return new FluentIterable<T>() {
      @Override
      public UnmodifiableIterator<T> iterator() {
        return postOrderIterator(root);
      }
    };
  }

  UnmodifiableIterator<T> postOrderIterator(T root) {
    return new PostOrderIterator(root);
  }

  private static final class PostOrderNode<T> {
    final T root;
    final Iterator<T> childIterator;