protected function selector()
    {
        $factor = null;

        // first factor
        if (!$factor = $this->is("factor")) {
            return false;
        }
        $selector = new CssParserModelSelector();
        $selector->addFactor($factor);

        // additional factors
        while ($factor = $this->is("factor")) {
            $selector->addFactor($factor);
        }

        return $selector;
    }