@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <U> Observable<U> concatMapIterable(final Function<? super T, ? extends Iterable<? extends U>> mapper, int prefetch) {
        ObjectHelper.requireNonNull(mapper, "mapper is null");
        ObjectHelper.verifyPositive(prefetch, "prefetch");
        return concatMap(ObservableInternalHelper.flatMapIntoIterable(mapper), prefetch);
    }