protected function _toSequenceString($sort = true)
    {
        /* $sort is ignored - see _sort(). */

        /* Use space as delimiter as it is the only printable ASCII character
         * that is not allowed as part of the UID (RFC 1939 [7]). */
        return implode(' ', count($this->_ids) > 25000 ? array_unique($this->_ids) : array_keys(array_flip($this->_ids)));
    }