docs: public static double pdf(double x, double k, double theta, double shift) {
      x = (x - shift);
      return x <= 0. ? 0. : FastMath.pow(theta, k) / GammaDistribution.gamma(k) * FastMath.pow(1 + x, -(theta + 1.)) * FastMath.pow(FastMath.log1p(x), k - 1.);
    }