docs: public void addScannerId(final int query_index, final int id, 
        final String string_id) {
      Map<Integer, String> scanners = scanner_ids.get(query_index);
      if (scanners == null) {
        scanners = new ConcurrentHashMap<Integer, String>();
        scanner_ids.put(query_index, scanners);
      }
      scanners.put(id, string_id);
    }