public static void Register(ICityNameObserver o)
 {
     city_observers.Add(o);
     if (HasProvider)
     {
         o.OnCityNameUpdated(provider.CityName);
     }
 }
 public static void Detach(ICityNameObserver o)
 {
     city_observers.Remove(o);
 }
 public static void Register(ICityNameObserver o)
 {
     city_observers.Add (o);
     if (HasProvider) {
         o.OnCityNameUpdated (provider.CityName);
     }
 }
 public static void Detach(ICityNameObserver o)
 {
     city_observers.Remove (o);
 }