* function} returns {@code null} for an input, it is replaced with an empty stream.)
   */
  static <InElementT extends @Nullable Object> Spliterator.OfLong flatMapToLong(
      Spliterator<InElementT> fromSpliterator,
      Function<? super InElementT, Spliterator.@Nullable OfLong> function,
      int topCharacteristics,
      long topSize) {
    checkArgument(

   * function} returns {@code null} for an input, it is replaced with an empty stream.)
   */
  static <InElementT extends @Nullable Object> Spliterator.OfLong flatMapToLong(
      Spliterator<InElementT> fromSpliterator,
      Function<? super InElementT, Spliterator.@Nullable OfLong> function,
      int topCharacteristics,
      long topSize) {
    checkArgument(

  /**
   * Returns a {@link LongStream} containing the elements of the first stream, followed by the
   * elements of the second stream, and so on.
   *
   * <p>This is equivalent to {@code Stream.of(streams).flatMapToLong(stream -> stream)}, but the
   * returned stream may perform better.
   *
   * @see LongStream#concat(LongStream, LongStream)
   */
  public static LongStream concat(LongStream... streams) {

  /**
   * Returns a {@link LongStream} containing the elements of the first stream, followed by the
   * elements of the second stream, and so on.
   *
   * <p>This is equivalent to {@code Stream.of(streams).flatMapToLong(stream -> stream)}, but the
   * returned stream may perform better.
   *
   * @see LongStream#concat(LongStream, LongStream)
   */
  public static LongStream concat(LongStream... streams) {

                    4))
        .expect(1, 1, 2, 3);
  }

  public void testFlatMapToLong_nullStream() {
    SpliteratorTester.ofLong(
            () ->
                CollectSpliterators.flatMapToLong(
                    Arrays.spliterator(new Long[] {1L, 0L, 1L, 2L, 3L}),
                    (Long i) -> i == 0L ? null : LongStream.of(i).spliterator(),

                    4))
        .expect(1, 1, 2, 3);
  }

  public void testFlatMapToLong_nullStream() {
    SpliteratorTester.ofLong(
            () ->
                CollectSpliterators.flatMapToLong(
                    Arrays.spliterator(new Long[] {1L, 0L, 1L, 2L, 3L}),
                    (Long i) -> i == 0L ? null : LongStream.of(i).spliterator(),