docs: @Override
    public void put(Mean other) {
      if(!(other instanceof MeanVariance)) {
        throw new IllegalArgumentException("I cannot combine Mean and MeanVariance to a MeanVariance.");
      }
      final MeanVariance mvo = (MeanVariance) other;
      final double on = mvo.n, osum = mvo.sum;
      final double tmp = n * osum - sum * on;
      final double oldn = n; // tmp copy
      n += on;
      sum += osum;
      m2 += mvo.m2 + tmp * tmp / (on * n * oldn);
    }