code: function(x) {
      var z = Math.abs(x);
      var t = 1 / (1 + z / 2);
      var r = t * Math.exp(-z * z - 1.26551223 + t * (1.00002368 +
              t * (0.37409196 + t * (0.09678418 + t * (-0.18628806 +
              t * (0.27886807 + t * (-1.13520398 + t * (1.48851587 +
              t * (-0.82215223 + t * 0.17087277)))))))))
      return x >= 0 ? r : 2 - r;
    }
docs: Complementary error function From Numerical Recipes in C 2e p221