Tdf.cs

        public static IObservable<TResult> FromTdf<T, TResult>(this IObservable<T> source,
            Func<IPropagatorBlock<T, TResult>> blockFactory) => Observable.Create<TResult>(observer => {
                var block = blockFactory();
                var dsp1 = block.AsObservable().Subscribe(observer.OnNext);
                var dsp2 = source.Subscribe(block.AsObserver());
                return new CompositeDisposable {dsp2, dsp1};
            });

        public static IObservable<TResult> FromTdf<T, TResult>(this IObservable<T> source,
            IPropagatorBlock<T, TResult> block) => source.FromTdf(() => block);