docs: public static double cdf(double x, double k, double theta, double shift) {
      if(x == Double.NEGATIVE_INFINITY) {
        return 0.;
      }
      if(x == Double.POSITIVE_INFINITY) {
        return 1.;
      }
      if(x != x) {
        return Double.NaN;
      }
      final double e = FastMath.exp((x - shift) * theta);
      return e < Double.POSITIVE_INFINITY ? GammaDistribution.regularizedGammaP(k, e) : 1.;
    }