public static void sumNegativeBalances() {
        // External iteration
        BigInteger total = BigInteger.ZERO;
        for (BankAccount account: christmasBank.getAllAccounts())
            if (account.getBalance().signum() < 0)
                total = total.add(account.getBalance());
        System.out.println("Total of negative balances (int): " + total);
        // Internal iteration
        BigInteger totalAlt = BigInteger.ZERO;
            // To be worked out.
        System.out.println("Total of negative balances (ext): " + totalAlt);
    }