public abstract void SendMessage(Colleague sender, string message);
 public abstract void SendMessage(Colleague sender, string message);
 public override void SendMessage(Colleague sender, string message)
 {
     if (sender == _colleague1)
       {
     _colleague2.Notify(message);
       }
       else if (sender == _colleague2)
       {
     _colleague1.Notify(message);
       }
 }