private function _mergeNorms()
    {
        foreach ($this->_writer->getFieldInfos() as $fieldInfo) {
            if ($fieldInfo->isIndexed) {
                foreach ($this->_segmentInfos as $segName => $segmentInfo) {
                    if ($segmentInfo->hasDeletions()) {
                        $srcNorm = $segmentInfo->normVector($fieldInfo->name);
                        $norm    = '';
                        $docs    = $segmentInfo->count();
                        for ($count = 0; $count < $docs; $count++) {
                            if (!$segmentInfo->isDeleted($count)) {
                                $norm .= $srcNorm[$count];
                            }
                        }
                        $this->_writer->addNorm($fieldInfo->name, $norm);
                    } else {
                        $this->_writer->addNorm($fieldInfo->name, $segmentInfo->normVector($fieldInfo->name));
                    }
                }
            }
        }
    }