Reesha: A2 - C1 - B2 is incorrect, but it leads to the same position as
the correct one (C2 - A3)
Ivan Detkov: Your move has been added. It solves the problem but I would never make such a move in a game because it is losing a very good ko-threat. It is aji keshi.
posetcay: I agree that it looks much better now, but B18 should also be considered correct, since black can get a sente seki on top of getting A16 in sente.
caranthir: I agree that that is the best endgame, unless White were globally very strong ko-wise. I added the seki variation with B19. If White were to start ko after that, they don't have a local threat like in the case Black had played C19.
posetcay: After A18, A17 is a wrong response. A18-C19 and its alive. As it stands, the problem is broken and must be fixed. It can be an endgame problem at best I think.
Ivan Detkov: Yes, this one is broken. I can just confirm what caranthir said already.
GoDave89: This one is unkillable (I remember spending aaaages on this one a few years ago :'D).
T2 is still correct, but only endgame, because it can be refuted by R1! (So T2-R1-T3-Q2-S2-R2)
caranthir: Oh, then this should be replaced or modified somehow.
A2 - C1 - B2 is incorrect, but it leads to the same position as the correct one (C2 - A3)