@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <R> Observable<R> concatMap(Function<? super T, ? extends ObservableSource<? extends R>> mapper, int prefetch) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        if (this instanceof ScalarCallable) {
            @SuppressWarnings("unchecked")
            T v = ((ScalarCallable<T>)this).call();
            if (v == null) {
                return empty();
            }
            return ObservableScalarXMap.scalarXMap(v, mapper);
        }
        return RxJavaPlugins.onAssembly(new ObservableConcatMap<T, R>(this, mapper, prefetch, ErrorMode.IMMEDIATE));
    }