supervlan mask snmp

sub vlanBelongsToSupervlan($svid, $vid) {
    my $low = (floor($vid/512)-1)*64 + 1;
    my $high = $low + 63;
    my $start = ($low-1)*8 + 1;
    my $shift = $vid - $start;
    my $mask = 1 << $shift;
    my $mib = "swSubVlanList$lowto$high.$svid";
    $value = snmpget($mib); #XXX
    return ($value & $mask)? 1 : 0;
}