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 (ext): " + total);
        // Internal iteration
        Optional<BigInteger> totalAlt = christmasBank.getAllAccounts().stream().
                filter(account -> account.getBalance().signum() < 0).
                map(account -> account.getBalance()).
                reduce((BigInteger left, BigInteger right) -> left.add(right));
        System.out.println("Total of negative balances (int): " + total);
    }