public void Perform(Collections.ObservableHashSet <TItem> target)
 {
     if (_keyResolved)
     {
         if (target.Contains(_key))
         {
             target.Remove(_key);
         }
     }
 }
 public override void Rollback(Collections.ObservableHashSet <TItem> target)
 {
     target.Remove(_addedItem);
 }
 public override void Perform(Collections.ObservableHashSet <TItem> target)
 {
     target.Remove(_item);
 }