private function checksum($code)
	{
		$len = strlen($code);
		$sum = 0;
		for ($i = 0; $i < $len; $i += 2) {
			$sum += $code[$i];
		}
		$sum *= 3;
		for ($i = 1; $i < $len; $i += 2) {
			$sum += ($code[$i]);
		}
		$r = $sum % 10;
		if ($r > 0) {
			$r = (10 - $r);
		}
		return $r;
	}