class SingleFlatMapSingle extends CompletionSingle<R> {

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

      SingleFlatMapSingle(Single<T> source, Function<? super T, ? extends Single<? extends R>> mapper);

}