public override void NotifyAllRemoval()
 {
     for (int i = 0; i < Observers.Count; i++)
     {
         PersonObserver p = Observers[i] as PersonObserver;
         p.NotifyRemoval(this);
     }
 }
 public override void NotifyAllChange()
 {
     for (int i = 0; i < Observers.Count; i++)
     {
         PersonObserver p = Observers[i] as PersonObserver;
         p.NotifyChange(this);
     }
 }