public void An_unsubscribed_does_not_receive_the_Put_element()
{
var s = new CountableStack <int>(3);
var sub1 = new StackSubscriber <int>();
var sub2 = new StackSubscriber <int>();
s.Subscribe(sub1);
s.Subscribe(sub2);
s.Put(40);
s.Unsubscribe(sub1);
s.Put(41);
Assert.AreEqual(40, sub1.LastPut);
Assert.AreEqual(41, sub2.LastPut);
}