public function mergeRulesets($keys)
    {
        $keys = is_array($keys) ? $keys : func_get_args();
        $rulesets = [];
        foreach ($keys as $key) {
            $rulesets[] = (array) $this->getRuleset($key, false);
        }

        return array_filter(call_user_func_array('array_merge', $rulesets));
    }