public function fetch(/* $key1, $key2, ... $keyN */)
    {
        $keys = func_get_args();

        $config = $this->config;
        foreach ($keys as $key) {
            if (isset($config[$key])) {
                $config = $config[$key];
            } else {
                throw new KeyNotFoundException("Key $key not found.");
            }
        }

        return $config;
    }