def zip(*iterables): #zip('ABCD','xy')-->AxBy sentinel=object() iterators=[iter(it)foritiniterables] while iterators: result=[] for it in iterators: elem=next(it,sentinel) if elem is sentinel: return result.append(elem) yieldtuple(result)这个第三行和第四行的代码怎么理解啊，尤其是第四行的，看不明白。谢谢啦