public IDisposable Subscribe(IObserver <T> observer)
        {
            var parent = new FromObserver(observer);

            source.Subscribe(parent);
            return(parent);
        }
        public void Subscribe(ISignalObserver <T> observer)
        {
            var parent = new FromObserver(observer);

            observer.OnSubscribe(parent);

            parent.OnSubscribe(source.Subscribe(parent));
        }