*
         * (For further discussion of nullness, see the comment above the method.)
         */
        return requireNonNull(value);
      }
    }
    OptionalState state = new OptionalState();

    Deque<Spliterator<T>> splits = new ArrayDeque<>();
    splits.addLast(stream.spliterator());

    while (!splits.isEmpty()) {
      Spliterator<T> spliterator = splits.removeLast();