code: public Flowable<Tree<T>> parents() {
      return Flowable.generate(() -> Wrapper.of(this), (n, e) -> {
        Tree<T> parent = n.get().getParent();
        if (parent != null) {
          e.onNext(parent);
          n.set(parent);
        } else {
          e.onComplete();
        }
      });
    }
docs: Get all of the node's parent
  
  @return