docs: private boolean sortedAndUnique(String[] keyList, int size)
        throws IOException {
      if (!closed) {
        for (int i = 1; i < size; i++) {
          if (keyList[(i - 1)].compareTo(keyList[i]) >= 0) {
            return false;
          }
        }
        return true;
      } else {
        throw new IOException("already closed");
      }
    }