docs: public static double pdf(double x, double mu, double sigma, double lambda) {
      final double dx = x - mu;
      final double lss = lambda * sigma * sigma;
      final double erfc = NormalDistribution.erfc((lss - dx) / (sigma * MathUtil.SQRT2));
      return erfc > 0. ? .5 * lambda * FastMath.exp(lambda * (lss * .5 - dx)) * erfc : (x == x) ? 0. : Double.NaN;
    }