docs: @Override
    public void put(double val) {
      if(n <= 0) {
        n = 1;
        sum = val;
        m2 = 0;
        return;
      }
      final double tmp = n * val - sum;
      final double oldn = n; // tmp copy
      n += 1.0;
      sum += val;
      m2 += tmp * tmp / (n * oldn);
    }