code: File findFinalizedEditsFile(long startTxId, long endTxId) throws IOException {
      File ret = new File(sd.getCurrentDir(),
          NNStorage.getFinalizedEditsFileName(startTxId, endTxId));
      if (!ret.exists()) {
        throw new IOException(
            "No edits file for range " + startTxId + "-" + endTxId);
      }
      return ret;
    }
docs: Find an edits file spanning the given transaction ID range.
  If no such file exists, an exception is thrown.