docs: public static double pdf(double x, double k, double lambda, double theta) {
      if(x <= theta || x == Double.POSITIVE_INFINITY) {
        return 0.;
      }
      double xl = (x - theta) / lambda;
      double p = FastMath.pow(xl, k - 1), p2 = p * -xl;
      return p2 != Double.NEGATIVE_INFINITY ? k / lambda * p * FastMath.exp(p2) : 0.;
    }