public static void Main (string[] args)
		{
			var sender = new Sender ();
			var dog = new Dog (sender);
			var cat = new Cat (sender);

			sender.StartTimer (1000, "Before unregister");

			dog.Unregister (sender);
			sender.StartTimer (2000, "After unregister");
		}
		public void Unregister (Sender sender)
		{
			sender.Mail -= Voice;
		}
		public Dog (Sender sender)
		{
			sender.Mail += Voice;
		}
		public Cat (Sender sender)
		{
			sender.Mail += Voice;
		}