docs: public static double pdf(double val, double loc, double scale, double shape) {
      val = (val - loc) / scale;
      if(shape != 0.) {
        val = -FastMath.log(1 - shape * val) / shape;
      }
      double f = 1. + FastMath.exp(-val);
      return FastMath.exp(-val * (1 - shape)) / (scale * f * f);
    }