public Either <Cart, Error> Add_(Guid productId, int quantity, TemporaryReservation temporaryReservation)
 {
     // NB: for sure we could check that before, but in real, we should test current cart nb + quantity added here...
     if (quantity > 99)
     {
         return(Either <Cart, Error> .Right(new Error("Too many of the same product in cart")));
     }
     // modify this
     // OR probably a new cart instance instead of modifying this...
     return(Either <Cart, Error> .Left(this));
 }
 // bool return type is only relevant with CartControllerIfs implementation
 public bool Add(Guid productId, int quantity, TemporaryReservation temporaryReservation)
 {
     throw new NotImplementedException();
 }