public override void Add(BaseElement element)
        {
            Pair pairNew = element as Pair;

            // check existsing item and remove it
            foreach (Pair item in Items)
            {
                if (item.Key == pairNew.Key)
                {
                    Items.Remove(item);
                    break;
                }
            }

            // add
            Items.Add(pairNew);
        }
 // Methods
 public abstract void Add(BaseElement element);