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