docs: public double getSampleExcessKurtosis() {
      if(!(m2 > 0) || !(n > 3)) {
        throw new ArithmeticException("Kurtosis not defined when variance is 0 or weight <= 3.0!");
      }
      final double nm1 = n - 1.;
      return (nm1 / ((n - 2.) * (n - 3.))) * (n * (n + 1) * m4 / (m2 * m2) - 3 * nm1);
    }