public void Update(CheckingAccount account)
        {
            CheckingAccount checkingAccount = Accounts.Single(a => a.Id == account.Id);

            checkingAccount.Description = account.Description;
        }
 public void Add(CheckingAccount account)
 {
     int max = Accounts.Max(a => a.Id);
     account.Id = max + 1;
     Accounts.Add(account);
 }