Osamu09: I'm actually very confused as well. After comparing the 2 sequences I can't see any reason why starting the ko would be bad. White has no local ko threat
Ivan Detkov: A18 is even better. I fixed the problem.
tenuki__: Phrase "In all problems, the player must either live or kill unconditionally. An answer that leads to a ko is a failure." in the description of the collection confused me when I tried to solve this problem.
A18 is even better. I fixed the problem.