@SuppressWarnings({ "unchecked", "rawtypes" })
    @CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.NONE)
    public static <T> Observable<T> concat(Iterable<? extends ObservableSource<? extends T>> sources) {
        ObjectHelper.requireNonNull(sources, "sources is null");
        return fromIterable(sources).concatMapDelayError((Function)Functions.identity(), bufferSize(), false);
    }