public override void RemoveItem(Stationary item)
 {
     items.Remove(item);
 }
 public override void AddItem(Stationary item)
 {
     items.Add(item);
 }
 public override void RemoveItem(Stationary item)
 {
 }
 public override void AddItem(Stationary item)
 {
 }
 public abstract void RemoveItem(Stationary item);
 public abstract void AddItem(Stationary item);