@SuppressWarnings({ "unchecked", "rawtypes" })
    @CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Observable<T> concat(ObservableSource<? extends ObservableSource<? extends T>> sources, int prefetch) {
        ObjectHelper.requireNonNull(sources, "sources is null");
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return RxJavaPlugins.onAssembly(new ObservableConcatMap(sources, Functions.identity(), prefetch, ErrorMode.IMMEDIATE));
    }