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