private function _loadPre21DelFile()
    {
        require_once 'Zend/Search/Lucene/Exception.php';
        try {
            // '.del' files always stored in a separate file
            // Segment compound is not used
            $delFile = $this->_directory->getFileObject($this->_name . '.del');

            $byteCount = $delFile->readInt();
            $byteCount = ceil($byteCount/8);
            $bitCount  = $delFile->readInt();

            if ($bitCount == 0) {
                $delBytes = '';
            } else {
                $delBytes = $delFile->readBytes($byteCount);
            }

            if (extension_loaded('bitset')) {
                return $delBytes;
            } else {
                $deletions = array();
                for ($count = 0; $count < $byteCount; $count++) {
                    $byte = ord($delBytes[$count]);
                    for ($bit = 0; $bit < 8; $bit++) {
                        if ($byte & (1<<$bit)) {
                            $deletions[$count*8 + $bit] = 1;
                        }
                    }
                }

                return $deletions;
            }
        } catch(Zend_Search_Lucene_Exception $e) {
            if (strpos($e->getMessage(), 'is not readable') === false) {
                throw new Zend_Search_Lucene_Exception($e->getMessage(), $e->getCode(), $e);
            }
            // There is no deletion file
            $this->_delGen = -1;

            return null;
        }
    }