code: public void addAll(PairedStats values) {
      if (values.count() == 0) {
        return;
      }
  
      xStats.addAll(values.xStats());
      if (yStats.count() == 0) {
        sumOfProductsOfDeltas = values.sumOfProductsOfDeltas();
      } else {
        // This is a generalized version of the calculation in add(double, double) above. Note that
        // non-finite inputs will have sumOfProductsOfDeltas = NaN, so non-finite values will result
        // in NaN naturally.
        sumOfProductsOfDeltas +=
            values.sumOfProductsOfDeltas()
                + (values.xStats().mean() - xStats.mean())
                    * (values.yStats().mean() - yStats.mean())
                    * values.count();
      }
      yStats.addAll(values.yStats());
    }
docs: Adds the given statistics to the dataset, as if the individual values used to compute the
  statistics had been added directly.