docs: public static double pdf(double x, double mu, double sigma, double skew) {
      if(x != x) {
        return Double.NaN;
      }
      x = (x - mu) / sigma; // Scale
      if(skew == 0.) {
        return MathUtil.ONE_BY_SQRTTWOPI / sigma * FastMath.exp(-.5 * x * x);
      }
      final double y = -FastMath.log1p(-skew * x) / skew;
      if(y != y || y == Double.POSITIVE_INFINITY || y == Double.NEGATIVE_INFINITY) { // NaN
        return 0.;
      }
      return MathUtil.ONE_BY_SQRTTWOPI / sigma * FastMath.exp(-.5 * y * y) / (1 - skew * x);
    }