docs: public static double logpdf(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 ? FastMath.log(.5 * lambda * erfc) + lambda * (lss * .5 - dx) : (x == x) ? Double.NEGATIVE_INFINITY : Double.NaN;
    }