public ViewResult Checkout(Cart cart, ShippingDetails details)
 {
     if(cart.Lines.Count() == 0)
     {
         ModelState.AddModelError("", "Sorry, your cart is empty!");
     }
     if (ModelState.IsValid)
     {
         orderProcessor.ProcessOrder(cart, details);
         cart.Clear();
         return View("Completed");
     }
     else
     {
         return View(details);
     }
 }
 public void ProcessOrder(Cart cart, ShippingDetails shippingDetails)
 {
     // Lets pretend this is implemented
 }