code: public static double logcdf(double x, double k, double theta, double shift) {
      final double e = FastMath.exp((x - shift) * theta);
      return e < Double.POSITIVE_INFINITY ? GammaDistribution.logregularizedGammaP(k, e) : 0.;
    }
docs: The log CDF, static version.
  
  @param x Value
  @param k Shape k
  @param theta Theta = 1.0/Beta aka. "scaling" parameter
  @return cdf value