int mytest3(SV* sv_vec, unsigned int bit) {

    STRLEN vecbytes; // Length of vector in bytes
    unsigned __int64 *vec = (unsigned __int64 *) SvPV(sv_vec, vecbytes);

    if( bit / 8 >= vecbytes) return 0; // Check in range

    vec[ bit / 64] |= ( 1ULL << ( bit % 64 ) ); // Set bit (CHANGES $vector)

    return 1;
}

##</code><code>##

#define REGSIZE 64
#define TSTVEC( v, b ) v[ b / REGSIZE ] &  ( 1ULL << ( b % REGSIZE ) ) )
#define SETVEC( v, b ) v[ b / REGSIZE ] |= ( 1ULL << ( b % REGSIZE ) ) )