sub elfProef ($) {
        my ($account) = @_;
        die("elfproef: `$account' isn't 9 characters long\n")
                unless length($account) == 9;

        my $sum = 0;
        foreach (reverse(-9..-1)) {
                $sum += (substr($account, $_, 1) * abs($_));
        }

        return not ($sum % 11);
}