private boolean filesExist(SegmentInfos toSync) throws IOException {
    
    Collection<String> files = toSync.files(false);
    for(final String fileName: files) {
      assert slowFileExists(directory, fileName): "file " + fileName + " does not exist; files=" + Arrays.toString(directory.listAll());
      // If this trips it means we are missing a call to
      // .checkpoint somewhere, because by the time we
      // are called, deleter should know about every
      // file referenced by the current head
      // segmentInfos:
      assert deleter.exists(fileName): "IndexFileDeleter doesn't know about file " + fileName;
    }
    return true;
  }