private function findRuleClass($nid) {
        if (!isset($this->rules[$nid])) {
            return null;
        } elseif (!empty($this->rules[$nid]['aliasOf'])) {
            return $this->findRuleClass($this->rules[$nid]['aliasOf']);
        } elseif (!empty($this->rules[$nid]['class'])) {
            return $this->rules[$nid]['class'];
        }

        return null;
    }