}

  private abstract static class MapWithIndexSpliterator<
          F extends Spliterator<?>,
          R extends @Nullable Object,
          S extends MapWithIndexSpliterator<F, R, S>>
      implements Spliterator<R> {
    final F fromSpliterator;
    long index;

    MapWithIndexSpliterator(F fromSpliterator, long index) {
      this.fromSpliterator = fromSpliterator;