docs: public static double logpdf(double x, double k, double theta, double shift) {
      x = (x - shift);
      if(x <= 0.) {
        return Double.NEGATIVE_INFINITY;
      }
      final double log1px = FastMath.log1p(x);
      return k * FastMath.log(theta) - GammaDistribution.logGamma(k) - (theta + 1.) * log1px + (k - 1) * FastMath.log(log1px);
    }