code: public static double cdf(double val, double k, double theta) {
      if(val < 0) {
        return 0.;
      }
      final double vt = val * theta;
      return (vt == Double.POSITIVE_INFINITY) ? 1. : regularizedGammaP(k, vt);
    }
docs: The CDF, static version.
  
  @param val Value
  @param k Shape k
  @param theta Theta = 1.0/Beta aka. "scaling" parameter
  @return cdf value