simpleTom: I think the topic of the last move could be avoided with a comment/question, if Tenuki is possible or another move is required. I made a proposal.
Sadaharu: Isn't it already thousand-year ko up to the sequence of T19 atari? Why need to continue playing after that...?
caranthir: I think that playing elsewhere at some odd point, after forming the mannen ko, is in this problem (and in at least one other) a way to indicate that the player "understands" that the position should be left as is. Other way would be some kind of pass or "tenuki" button, but those are not implemented in Tsumegohero. Moreover, every correct sequence in Tsumegohero must end in player's move. Within the framework at hand, the final required move should be here either the capture at T18, or a "placeholder" move after opponent's subsequent ate, as is done here.
thepope: The last move is really confusing, how can you tell which moves are valid "passes"? Bottom left orientation, J3 is not valid, and I suspect many others are not valid as well. There's no "natural" move locally. I think either every visible intersection should be valid, or maybe add a "natural" tenuki like a capture or something
Careenium: I know this is a one thousand years ko, but I have no idea how to tell that to the problem in the last move. Isn't the best move to not play there?
StephaneLamy: It is strange that S13 is accepted as good answer for the last move, but not T15.
Seems the site would need a "pass" or "tenuki" option.
Or add an indication "use S13 if you wish to tenuki"?
totbeibrot: thats a 1000-years-ko. black shouldnt start the ko and instead tenuki
Farkas: I agree, it's a mannen-ko (10.000 year ko), and better for B to tenuki. Actually, playing inside is a mistake, therefore I added sequences and changed the ending.
Great! I'll delete our related messages from the other problem, so as to not spoil the "fun" from others :)