sub arabic {
    my($arg) = shift;
    isroman $arg or return undef;
    my($last_digit) = 1000;
    my($arabic);
    foreach (split(//, uc $arg)) {
        my($digit) = $roman2arabic{$_};
        $arabic -= 2 * $last_digit if $last_digit < $digit;
        $arabic += ($last_digit = $digit);
    }
    $arabic;
}