public void AddedIOrderItemsAppearInItemsProperty()
        {
            var order = new Order();
            var item  = new MockOrderItem();

            order.Add(item);
            Assert.Contains(item, order.Items);
        }
        public void RemovedOrderItemDoesNotAppearInItemsProperty()
        {
            var order = new Order();
            var item  = new MockOrderItem();

            order.Add(item);
            order.Remove(item);
            Assert.DoesNotContain(item, order.Items);
        }