* @since 24.1
   */
  public final Iterable<N> depthFirstPostOrder(Iterable<? extends N> startNodes) {
    ImmutableSet<N> validated = validate(startNodes);
    return () -> newTraversal().postOrder(validated.iterator());
  }

  abstract Traversal<N> newTraversal();

  @SuppressWarnings("CheckReturnValue")
  private ImmutableSet<N> validate(Iterable<? extends N> startNodes) {
    ImmutableSet<N> copy = ImmutableSet.copyOf(startNodes);