public void Unregister(AbstractObserver observer)
 {
     _observers.Remove(observer);
 }
 public void Register(AbstractObserver observer)
 {
     _observers.Add(observer);
 }
 public void Register(AbstractObserver observer)
 {
     _observers.Add(observer);
 }
 public void Unregister(AbstractObserver observer)
 {
     _observers.Remove(observer);
 }