public void Unregister(BankEvent be)
 {
     be.NewStock -= UpdateBank;
 }
 public Bank(BankEvent be)
 {
     be.NewStock += UpdateBank;
 }
 public void Register(BankEvent be)
 {
     be.NewStock += UpdateBank;
 }
 public Broker(BankEvent be)
 {
     be.NewStock += UpdateBroker;
 }