@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <U, R> Maybe<R> flatMap(Function<? super T, ? extends MaybeSource<? extends U>> mapper,
            BiFunction<? super T, ? super U, ? extends R> resultSelector) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        ObjectHelper.requireNonNull(resultSelector, "resultSelector is null");
        return RxJavaPlugins.onAssembly(new MaybeFlatMapBiSelector<T, U, R>(this, mapper, resultSelector));
    }