public static CustomerList operator -(CustomerList customerList, Customer customer)
 {
     CustomerList newCustomerList = new CustomerList(customerList);
     newCustomerList.customers.Remove(customer);
     newCustomerList.Changed(newCustomerList);
     return newCustomerList;
 }
 public CustomerList(CustomerList customerList)
 {
     this.customers = new List<Customer>(customerList.customers);
 }