protected static function _explodeDottedOID(string $oid): array
    {
        $subids = [];
        if (strlen($oid)) {
            foreach (explode(".", $oid) as $subid) {
                $n = @gmp_init($subid, 10);
                if (false === $n) {
                    throw new \UnexpectedValueException(
                        "'$subid' is not a number.");
                }
                $subids[] = $n;
            }
        }
        return $subids;
    }