@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> flatMapSingle(Function<? super T, ? extends SingleSource<? extends R>> mapper, boolean delayErrors) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        return RxJavaPlugins.onAssembly(new ObservableFlatMapSingle<T, R>(this, mapper, delayErrors));
    }