public Set<BankAccount> getAccountsBalanceNotBelow(BigInteger threshold) throws IllegalArgumentException {
        if (threshold == null)
            throw new IllegalArgumentException();
        Set<BankAccount> result = new HashSet<>();
        for (BankAccount account: accounts)
            if (account.getBalance().compareTo(threshold) >= 0)
                result.add(account);
        return result;
    }