public void Dispose()
        {
            var d = default(IDisposable);

            var us = new UnicastSubject <int>();

            var o = us.Cache(capacityHint: 16, cancel: v => d = v);

            Assert.IsNull(d);
            Assert.False(us.HasObserver());

            var to = o.Test();

            Assert.IsNotNull(d);
            Assert.True(us.HasObserver());

            d.Dispose();

            Assert.False(us.HasObserver());

            to.AssertFailure(typeof(OperationCanceledException));
        }