protected function _getInfoString(&$flags)
    {
        // accessing keys is easier, faster and it removes duplicated flags
        $wanted_flags = array_flip($flags);
        if (isset($wanted_flags[Zend_Mail_Storage::FLAG_RECENT])) {
            /**
             * @see Zend_Mail_Storage_Exception
             */
            // require_once 'Zend/Mail/Storage/Exception.php';
            throw new Zend_Mail_Storage_Exception('recent flag may not be set');
        }

        $info = ':2,';
        $flags = array();
        foreach (Zend_Mail_Storage_Maildir::$_knownFlags as $char => $flag) {
            if (!isset($wanted_flags[$flag])) {
                continue;
            }
            $info .= $char;
            $flags[$char] = $flag;
            unset($wanted_flags[$flag]);
        }

        if (!empty($wanted_flags)) {
            $wanted_flags = implode(', ', array_keys($wanted_flags));
            /**
             * @see Zend_Mail_Storage_Exception
             */
            // require_once 'Zend/Mail/Storage/Exception.php';
            throw new Zend_Mail_Storage_Exception('unknown flag(s): ' . $wanted_flags);
        }

        return $info;
    }