init
aiter
anext
f start: 20
coro yielded: 21
coro yielded: 22
f returned: 23
x 0
anext
f start: 20
coro yielded: 21
coro yielded: 22
f returned: 23
x 1
anext
f start: 20
coro yielded: 21
coro yielded: 22
f returned: 23
x 2
anext
f start: 20
coro yielded: 21
coro yielded: 22
f returned: 23
x 3
anext
f start: 20
coro yielded: 21
coro yielded: 22
f returned: 23
finished