docs: public static double logpdf(double x, double mu, double sigma, double skew) {
      if(x != x) {
        return Double.NaN;
      }
      x = (x - mu) / sigma;
      if(skew == 0.) {
        return MathUtil.LOG_ONE_BY_SQRTTWOPI - FastMath.log(sigma) - .5 * x * x;
      }
      double y = -FastMath.log(1. - skew * x) / skew;
      if(y != y || y == Double.NEGATIVE_INFINITY || y == Double.NEGATIVE_INFINITY) {
        return Double.NEGATIVE_INFINITY;
      }
      return -.5 * y * y - FastMath.log(MathUtil.ONE_BY_SQRTTWOPI * sigma * (1 - skew * x));
    }