public static class CartExtensions
    {
        public static Cart<TItem> Add<TItem>(this Cart<TItem> cart, TItem item)
            => cart.Match<Cart<TItem>, Cart<TItem>>()
                .With<EmptyCart<TItem>>(x => x.AddItem(item))
                .With<ActiveCart<TItem>>(x => x.AddItem(item))
                .With<PaidCart<TItem>>(x =>
                {
                    Console.WriteLine("ERROR: The cart is paid for and items cannot be added");
                    return x;
                })
                .Return();

        public static Cart<TItem> Remove<TItem>(this Cart<TItem> cart, TItem item)
            => cart.Match<Cart<TItem>, Cart<TItem>>()
                .With<EmptyCart<TItem>>(x =>
                {
                    Console.WriteLine("ERROR: The cart is empty and items cannot be removed");
                    return x;
                })
                .With<ActiveCart<TItem>>(x => x.RemoveItem(item))
                .With<PaidCart<TItem>>(x =>
                {
                    Console.WriteLine("ERROR: The cart is paid for and items cannot be removed");
                    return x;
                })
                .Return();

        public static void Display<TItem>(this Cart<TItem> cart)
            => cart.Match<Cart<TItem>, object>()
                .With<EmptyCart<TItem>>(x =>
                {
                    Console.WriteLine("The cart is empty");
                    return null;
                })
                .With<ActiveCart<TItem>>(x =>
                {
                    Console.WriteLine("The active cart contains {0} items", x.Items.Count());
                    return null;
                })
                .With<PaidCart<TItem>>(x =>
                {
                    Console.WriteLine("The paid cart contains {0} items. Amount paid {1}");
                    return null;
                })
                .Return();
    }

static void Main(string[] args)
        {
            var emptyCart = new EmptyCart<string>();
            emptyCart.Display();

            var cartA = emptyCart.Add("A");  //one item
            emptyCart.Add("A").Display();

            var cartAb = cartA.Add("B");  //two items
            cartAb.Display();

            var cartB = cartAb.Remove("A"); //one item
            cartB.Display();

            var emptyCart2 = cartB.Remove("B"); //empty
            emptyCart2.Display();

            Console.WriteLine("Removing from emptyCart");
            emptyCart.Remove("B"); //error

            //  try to pay for cartA

            Console.WriteLine("paying for cartA");
            var paidCart = cartA.Match<Cart<string>, Cart<string>>()
               .With<EmptyCart<string>>(x => x)
               .With<ActiveCart<string>>(x => x.Pay(100))
               .With<PaidCart<string>>(x => x)
               .Return();

            paidCart.Display();

            Console.WriteLine("paying for emptyCart");
            var emptyCartPaid = emptyCart.Match<Cart<string>, Cart<string>>()
                .With<EmptyCart<string>>(x => emptyCart)
                .With<ActiveCart<string>>(x => x.Pay(100))
                .With<PaidCart<string>>(x => emptyCart)
                .Return();


            emptyCartPaid.Display();
        }