public function setPriority($priority = null)
    {
        if (!$priority) {
            return $this;
        }

        if ($priority && is_numeric($priority) && $priority >= 0 && $priority <= 1) {
            $this->priority = number_format($priority, 1);
        } else {
            throw new \RuntimeException(
                sprintf(
                    'The value "%s" is not supported by the option priority, it must be a numeric between 0.0 and 1.0. See http://www.sitemaps.org/protocol.html#xmlTagDefinitions',
                    $priority
                )
            );
        }

        return $this;
    }