1init 2aiter 3anext 4f start: 20 5coro yielded: 21 6coro yielded: 22 7f returned: 23 8x 0 9anext 10f start: 20 11coro yielded: 21 12coro yielded: 22 13f returned: 23 14x 1 15anext 16f start: 20 17coro yielded: 21 18coro yielded: 22 19f returned: 23 20x 2 21anext 22f start: 20 23coro yielded: 21 24coro yielded: 22 25f returned: 23 26x 3 27anext 28f start: 20 29coro yielded: 21 30coro yielded: 22 31f returned: 23 32finished 33