understand co (a simple co)

co.js

function co(GenFunc) {
  return function(cb) {
    var gen = GenFunc()
    next()
    function next(err, args) {
      if (err) {
        cb(err)
      } else {
        if (gen.next) {
          var ret = gen.next(args)
          if (ret.done) {
            // console.log(cb, args);
            cb && cb(null, args)
          } else {
            ret.value(next)
          }
        }
      }
    }
  }
}

co(function *(){
  var res = yield (function(){
    return function(cb){
      cb(null, 1, 2)
    }
  })();

  console.log('res: ', res);  // res: 1

})(function(){
  console.log('args: ', arguments);  // args: [null, 1]
});