private bool HasConcept(Concept concept)
 {
     throw new NotImplementedException();
 }
 private void RemoveConcept(Concept concept)
 {
     Items.Remove(concept);
     DoRemovedProp(concept);
 }
 private void AddConcept(Concept concept)
 {
     Items.Add(concept);
     DoAddProp(concept);
 }