public function isOptivumTimetable(string $html) : bool
    {
        if (strpos($html, 'Plan lekcji Optivum firmy VULCAN') !== false) {
            return true;
        }

        libxml_use_internal_errors(true);
        $dom = new DOMDocument();
        $dom->loadHTML($html);
        $frames = $dom->getElementsByTagName('frame');

        /** @var \DOMElement $node */
        foreach ($frames as $node) {
            if ('lista.html' === $node->getAttribute('src')) {
                return true;
            }
        }

        return false;
    }