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

  }

  /**
   * Returns an {@link IntStream} 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).flatMapToInt(stream -> stream)}, but the
   * returned stream may perform better.
   *
   * @see IntStream#concat(IntStream, IntStream)
   */
  public static IntStream concat(IntStream... streams) {

  }

  /**
   * Returns an {@link IntStream} 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).flatMapToInt(stream -> stream)}, but the
   * returned stream may perform better.
   *
   * @see IntStream#concat(IntStream, IntStream)
   */
  public static IntStream concat(IntStream... streams) {

                    7))
        .expect('a', 'b', 'c', 'd', 'e', 'f', 'g');
  }

  public void testFlatMapToInt_nullStream() {
    SpliteratorTester.ofInt(
            () ->
                CollectSpliterators.flatMapToInt(
                    Arrays.spliterator(new Integer[] {1, 0, 1, 2, 3}),
                    (Integer i) -> i == 0 ? null : IntStream.of(i).spliterator(),