public void Remove(IGameObject2D gameObject)
 {
     // Implementation omitted.
 }
 public IEnumerable <IGameObject2D> FindGameObjectsAround(IGameObject2D target, int radius)
 {
     // Implementation omitted.
     return(Enumerable.Empty <IGameObject2D>());
 }
 public void Add(IGameObject2D gameObject)
 {
     // Implementation omitted.
 }