public void MembershipAdded()
        {
            var moqCustomer = new Mock <ICustomer>();
            var moqCR       = new Mock <ICustomerRetriever>();

            moqCR.Setup(x => x.RetrieveCustomer(It.IsAny <int>())).Returns(moqCustomer.Object);
            var moqPO = new PurchaseOrder("Customer: 123456", moqCR.Object, moqRefererRetriever.Object);

            moqPO.AddMembership(MembershipType.Books);
            moqCustomer.Verify(x => x.AddMembership(MembershipType.Books));
        }