caranthir: I updated variations with the premise that we should create ko in the best way. Q9 is the only accepted starting move. Notice that in the sequence Q19P19S19T17T18S18, now we should definitely take the opportunity to fix the outside shape with T15.
caranthir: 1) S19 shouldn't be a valid starting move. Opponent lives with Q19. 2) Q19P19S19T17. The variation should end there "in real life" (see below). To play R19 after this isn't meaningful (let's make the opponent counter with T18 to demonstrate that they can create seki). So we play T18 instead, and let's make the opponent counter with S18 so it's mannen ko. Also let's make the problem hint as "ko", because otherwise even R18 instead of S18 would be easily at least equally good (opponent could live, but we would have have forcing moves outside that can be sente).
TherapieGo: [current position] + A18 Seki is not correct no ?
caranthir: Yeah, that would be a definite seki of course. The thing is that in this variation the opponent actually shouldn't play anything after R19. T18 is the only meaningful move they have and it's totally gote - and without it the only meaningful move we have in turn is T18 to create mannen ko, which isn't much.
I updated variations accordingly.