protected function collection_to_array($collection)
    {
        if ($collection instanceof self) {
            return $collection->toArray();

        } else if ($collection instanceof \Traversable) {
            return iterator_to_array($collection);

        } else if ($collection instanceof Arrayable) {
            return $collection->toArray();

        } else if ($collection instanceof Jsonable) {
            return json_decode($collection->toJson(), true);

        } else if (is_object($collection) || is_array($collection)) {
            return (array)$collection;

        } else {
            throw new \InvalidArgumentException(sprintf(
                '$collection has type %s, which is not collection-like',
                gettype($collection)
            ));
        }
    }