int indexNth( SV *haystack, SV*needle, int nth ) {
    STRLEN lh, ln, i, j;
    char *h = SvPV( haystack, lh );
    char *n = SvPV( needle, ln );

    for( i = 0; i < lh; ++i ) {
        for( j = 0; j < ln; ++j ) {
            if( h[ i + j ] != n[ j ] ) goto nomatch;
        }
        if( --nth == 0 ) return i;
        nomatch:;
    }
    return -1;
}