private static void ObserverDemo()
{
var ibm = new IBM("IBM", 120.00);
var investor1 = new Investor("Sorros");
var investor2 = new Investor("Berkshire");
ibm.StartWatching(investor1.Update);
ibm.StartWatching(investor2.Update);
ibm.UpdatePrice(120.10);
ibm.UpdatePrice(121.00);
ibm.StopWatching(investor1.Update);
ibm.UpdatePrice(120.50);
ibm.StopWatching(investor2.Update);
ibm.UpdatePrice(120.75);
}