private void handleChanged(TECChangedEventArgs obj)
 {
     if (obj.Sender is TECBid && obj.Value is TECTypical typ)
     {
         if (obj.Change == Change.Add)
         {
             if (typ.Instances.Count != 1)
             {
                 TypicalSystems.Add(typ);
             }
             else
             {
                 Singletons.Add(typ);
             }
         }
         else if (obj.Change == Change.Remove)
         {
             TypicalSystems.Remove(typ);
             Singletons.Remove(typ);
         }
     }
     else if (obj.Sender is TECTypical typParent && obj.Value is TECSystem)
     {
         if (typParent.Instances.Count == 1)
         {
             TypicalSystems.Remove(typParent);
             Singletons.Add(typParent);
         }
         else
         {
             Singletons.Remove(typParent);
             if (!TypicalSystems.Contains(typParent))
             {
                 TypicalSystems.Add(typParent);
             }
         }
     }
 }