public void Accept(OfferVisitor visitor)
 {
     visitor.VisitBronzeCreditCard(this);
 }
 public void Accept(OfferVisitor visitor)
 {
     visitor.VisitSilverCreditCard(this);
 }
 public void Accept(OfferVisitor visitor)
 {
     visitor.VisitGoldCreditCard(this);
 }