sub comp {
    return undef unless length $_[0] == length $_[1];
    my $where;
    for ( 0 .. length $_[0] ) {
        next if substr($_[0], $_, 2) eq substr($_[1], $_, 2);
        if ( substr($_[0], $_, 2) eq reverse substr($_[1], $_, 2) ) {
            return undef if defined $where;
            $where = $_;
        } else { 
            return undef; 
        }
    }
    return "+$where" if defined $where;
}