public function setChangefreq($changefreq = null)
    {
        $frequencies = [
            self::CHANGEFREQ_ALWAYS,
            self::CHANGEFREQ_HOURLY,
            self::CHANGEFREQ_DAILY,
            self::CHANGEFREQ_WEEKLY,
            self::CHANGEFREQ_MONTHLY,
            self::CHANGEFREQ_YEARLY,
            self::CHANGEFREQ_NEVER,
            null,
        ];
        if (!in_array($changefreq, $frequencies)) {
            throw new \RuntimeException(
                sprintf(
                    'The value "%s" is not supported by the option changefreq. See http://www.sitemaps.org/protocol.html#xmlTagDefinitions',
                    $changefreq
                )
            );
        }

        $this->changefreq = $changefreq;

        return $this;
    }