docs: private void mapAndRememberIndexFile(File index) {
          MappedFileReader idxFileReader = null;
          try {
              idxFileReader = new MappedFileReader(index);
              mappedIndexFileReader.add(idxFileReader);
              indexFiles.add(idxFileReader.map());
          } catch(IOException e) {
              logger.error("Error mmapping " + index, e);
          }
      }