docs: public static double logpdf(double x, double mu, double sigma, double xi) {
      x = (x - mu) / sigma;
      // Check support:
      if(x < 0 || (xi < 0 && x > -1. / xi)) {
        return Double.NEGATIVE_INFINITY;
      }
      if(xi == 0) {
        return Double.POSITIVE_INFINITY;
      }
      return ((xi == -1) ? 0. : FastMath.log(1 + xi * x) * (-1 / xi - 1)) - FastMath.log(sigma);
    }