private void Coalesce(INotifyReversableExpression <IModelElement> coalesceValue)
 {
     if (ReferencedElement == null)
     {
         ReferencedElement = coalesceValue;
     }
     else
     {
         ReferencedElement = Observable.Coalesce(ReferencedElement, coalesceValue);
     }
 }
 private void Coalesce(INotifyReversableExpression<object> coalesceValue)
 {
     if (Value == null)
     {
         Value = coalesceValue;
     }
     else
     {
         Value = Observable.Coalesce(Value, coalesceValue);
     }
 }