protocol BuyerSellerCreditCheck (role Buyer) {
    Buyer introduces Seller;

    Order from Buyer to Seller;

    Seller introduces CreditAgency;

    CheckCredit from Seller to CreditAgency;

    choice at CreditAgency {
        CreditOk from CreditAgency to Seller;
        Invoice from Seller to Buyer;
    } or {
        NoCredit from CreditAgency to Seller;
        Rejected from Seller to Buyer;
    }
}