public void RemoveSubOrdinate(CompositeEmployee subordinate)
 {
     if (SubOrdinates.Contains(subordinate))
     {
         SubOrdinates.Remove(subordinate);
     }
 }
 public void AddSubOrdinate(CompositeEmployee subordinate)
 {
     if (!SubOrdinates.Contains(subordinate))
     {
         SubOrdinates.Add(subordinate);
     }
 }