public function extend($name, Closure $callback)
        {
            if (count($this->_items)) {
                $collection = array();

                foreach ($this->_items as $item) {
                    if ($item instanceof Container) {
                        $item->fn($name, $callback);
                    }

                    array_push($collection, $item);
                }

                return new self($collection);
            }
            return $this;
        }