docs: public static double cdf(double x, double mu, double sigma, double skew) {
      x = (x - mu) / sigma;
      if(Math.abs(skew) > 0.) {
        double tmp = 1 - skew * x;
        if(tmp < 1e-15) {
          return (skew < 0.) ? 0. : 1.;
        }
        x = -FastMath.log(tmp) / skew;
      }
      return .5 + .5 * NormalDistribution.erf(x * MathUtil.SQRTHALF);
    }