class MultiFlatMapPublisher implements Multi<R> {

    @Override
    public void subscribe(Flow.Subscriber<? super R> subscriber) {
        Objects.requireNonNull(subscriber, "subscriber is null");
        source.subscribe(new FlatMapSubscriber<>(subscriber, mapper, maxConcurrency,
                prefetch, delayErrors));
    }

      MultiFlatMapPublisher(Multi<T> source,
                          Function<? super T, ? extends Flow.Publisher<? extends R>> mapper,
                          long maxConcurrency, long prefetch, boolean delayErrors);

}