@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final <U, R> Observable<R> zipWith(ObservableSource<? extends U> other,
            BiFunction<? super T, ? super U, ? extends R> zipper) {
        ObjectHelper.requireNonNull(other, "other is null");
        return zip(this, other, zipper);
    }