@Override
    public T nextImpl() {
      if (top != null) {
        JBIterable.from(top.iterable(tree)).map(ourNotNullizer::notNullize).addAllTo(queue);
        top = null;
      }
      if (queue.isEmpty()) return stop();
      top = P1.create(ourNotNullizer.nullize(queue.remove()));
      return top.node;
    }
@Override
    public T nextImpl()