private void appendVerificationRequest(StringBuilder req, VerificationRequest vr)
 {
     if (vr != null)
     {
         this.appendParam(req, "avsRequestCode", vr.isAvsEnabled() ?  vr.getAvsRequest().ToString() : null);
         this.appendParam(req, "cvv2RequestCode", vr.isCvv2Enabled() ? vr.getCvv2Request().ToString() : null);
     }
 }