docs: public static double logpdf(double x, double sigma) {
      if(!(x > 0.) || x == Double.POSITIVE_INFINITY) {
        return x == x ? Double.NEGATIVE_INFINITY : Double.NaN;
      }
      final double xs = x / sigma, xs2 = xs * xs;
      return xs2 < Double.POSITIVE_INFINITY ? FastMath.log(xs / sigma) - .5 * xs2 : Double.NEGATIVE_INFINITY;
    }