public function getSubtitle()
    {
        return $this->getCachedProperty(
            'subtitle',
            function () {
                // No REQUIRED — no exception.
                $element = $this->query('atom:subtitle', Node::SINGLE);

                /** @var Element $this */
                return $element ? $this->getExtensions()->parseElement($this, $element) : null;
            }
        );
    }