docs: @Override
    public MtasDataCollector<?, ?> add(String key, double valueSum, long valueN)
        throws IOException {
      if (key != null) {
        MtasDataCollector<?, ?> subCollector = add(key, false);
        setValue(newCurrentPosition, Double.valueOf(valueSum).longValue(), valueN,
            newCurrentExisting);
        return subCollector;
      } else {
        return null;
      }
    }