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]';
};