@SuppressWarnings("unchecked")
    public static <T, R> boolean tryScalarXMapSubscribe(Publisher<T> source,
            Subscriber<? super R> subscriber,
            Function<? super T, ? extends Publisher<? extends R>> mapper) {
        if (source instanceof Callable) {
            T t;

            try {
                t = ((Callable<T>)source).call();
            } catch (Throwable ex) {
                Exceptions.throwIfFatal(ex);
                EmptySubscription.error(ex, subscriber);
                return true;
            }

            if (t == null) {
                EmptySubscription.complete(subscriber);
                return true;
            }

            Publisher<? extends R> r;

            try {
                r = ObjectHelper.requireNonNull(mapper.apply(t), "The mapper returned a null Publisher");
            } catch (Throwable ex) {
                Exceptions.throwIfFatal(ex);
                EmptySubscription.error(ex, subscriber);
                return true;
            }

            if (r instanceof Callable) {
                R u;

                try {
                    u = ((Callable<R>)r).call();
                } catch (Throwable ex) {
                    Exceptions.throwIfFatal(ex);
                    EmptySubscription.error(ex, subscriber);
                    return true;
                }

                if (u == null) {
                    EmptySubscription.complete(subscriber);
                    return true;
                }
                subscriber.onSubscribe(new ScalarSubscription<R>(subscriber, u));
            } else {
                r.subscribe(subscriber);
            }

            return true;
        }
        return false;
    }