docs: public static double quantile(double val, double mu, double sigma, double xi) {
      if(val < 0.0 || val > 1.0) {
        return Double.NaN;
      }
      if(xi == 0.) {
        return mu - sigma * FastMath.log(1 - val);
      }
      return mu - sigma / xi * (1 - FastMath.pow(1 - val, -xi));
    }