/** Factory for constructing {@link FlatMapSpliterator} instances. */
    interface Factory<InElementT extends @Nullable Object, OutSpliteratorT extends Spliterator<?>> {
      OutSpliteratorT newFlatMapSpliterator(
          @CheckForNull OutSpliteratorT prefix,
          Spliterator<InElementT> fromSplit,
          Function<? super InElementT, @Nullable OutSpliteratorT> function,
          int splitCharacteristics,