WithLatestFrom

        public static IObservable<TResult> WithLatestFrom<TFirst, TSecond, TResult>(this IObservable<TFirst> first, IObservable<TSecond> second, Func<TFirst, TSecond, TResult> resultSelector)
        {
            if (first == null)
                throw new ArgumentNullException("first");
            if (second == null)
                throw new ArgumentNullException("second");
            if (resultSelector == null)
                throw new ArgumentNullException("resultSelector");

            return first.Publish(f =>
            {
                return second.Publish(s =>
                {
                    return Observable.CombineLatest(
                        f.SkipUntil(s),
                        s,
                        Tuple.Create)
                        .TakeUntil(f.Materialize().Where(n => n.Kind == NotificationKind.OnCompleted))
                        .DistinctUntilChanged(tp => tp.Item1)
                        .Select(tp => resultSelector(tp.Item1, tp.Item2));
                });
            });
        }