code: public static IntStream concat(IntStream... streams) {
      // TODO(lowasser): optimize this later
      return Stream.of(streams).flatMapToInt(stream -> stream);
    }
docs: 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)