protected function append_result($result)
    {
        $this->meta['count'] += $result->count();

        // append UIDs to global index
        $folder = $result->get_parameters('MAILBOX');
        $index  = array_map(function($uid) use ($folder) { return $uid . '-' . $folder; }, $result->get());

        $this->index = array_merge($this->index, $index);
    }