--- /dev/null
+module.exports = function (xs, fn) {
+ var res = [];
+ for (var i = 0; i < xs.length; i++) {
+ var x = fn(xs[i], i);
+ if (isArray(x)) res.push.apply(res, x);
+ else res.push(x);
+ }
+ return res;
+};
+
+var isArray = Array.isArray || function (xs) {
+ return Object.prototype.toString.call(xs) === '[object Array]';
+};