ImmutableSet<N> validated = validate(startNodes);
    return new Iterable<N>() {
      @Override
      public Iterator<N> iterator() {
        return newTraversal().postOrder(validated.iterator());
      }
    };
  }

  abstract Traversal<N> newTraversal();

  @SuppressWarnings("CheckReturnValue")
  private ImmutableSet<N> validate(Iterable<? extends N> startNodes) {