kovarex: Right, after checking again, it really looks like 3 move approach ko, not 2, so I updated the problem.
The terminology follows this logic: https://senseis.xmp.net/?ApproachKo
The idea is the 0 approach ko is normal ko, 1 move approach ko is, when you (or the opponent) need to do an extra move to start a normal ko, etc.
kovarex: A16B18B17
I have no idea why this could be wrong. As long as I can see black is safely alive after playhing this. It should either be correct variation, or refutation should be added.
Right, after checking again, it really looks like 3 move approach ko, not 2, so I updated the problem. The terminology follows this logic: https://senseis.xmp.net/?ApproachKo The idea is the 0 approach ko is normal ko, 1 move approach ko is, when you (or the opponent) need to do an extra move to start a normal ko, etc.