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

  private final class BreadthFirstIterator extends UnmodifiableIterator<T>
      implements PeekingIterator<T> {
    private final Queue<T> queue;

    BreadthFirstIterator(T root) {
      this.queue = new ArrayDeque<>();
      queue.add(root);
    }