public function clean($delay = self::CLEAN_DELAY)
    {
        $delay += self::CACHE_TIME;
        $query = $this->pdo->prepare(<<<SQL
DELETE FROM robotstxt__cache1
WHERE nextUpdate < (UNIX_TIMESTAMP() - :delay);
SQL
        );
        $query->bindValue('delay', $delay, \PDO::PARAM_INT);
        return $query->execute();
    }