docs: public static double cdf(double val, double mu, double sigma, double xi) {
      val = (val - mu) / sigma;
      // Check support:
      if(val < 0) {
        return 0.;
      }
      if(xi < 0 && val > -1. / xi) {
        return 1.;
      }
      return 1 - FastMath.pow(1 + xi * val, -1. / xi);
    }