code: public static double logcdf(double x, double k, double theta, double shift) {
      x = (x - shift);
      return x <= 0. ? -Double.NEGATIVE_INFINITY : GammaDistribution.logregularizedGammaP(k, FastMath.log1p(x) * theta);
    }
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