Spliterator<OutElementT> flatMap(
          Spliterator<InElementT> fromSpliterator,
          Function<? super InElementT, @Nullable Spliterator<OutElementT>> function,
          int topCharacteristics,
          long topSize) {
    checkArgument(
        (topCharacteristics & Spliterator.SUBSIZED) == 0,
        "flatMap does not support SUBSIZED characteristic");
    checkArgument(
        (topCharacteristics & Spliterator.SORTED) == 0,