public IDisposable Subscribe(IObserver <T> observer)
        {
            var en      = _source.GetAsyncEnumerator();
            var handler = new ToObservableHandler(observer, en);

            handler.MoveNext();
            return(handler);
        }
        public IDisposable Subscribe(IObserver <T> observer)
        {
            var cts     = new CancellationTokenSource();
            var en      = _source.GetAsyncEnumerator(cts.Token);
            var handler = new ToObservableHandler(observer, en, cts);

            handler.MoveNext();
            return(handler);
        }