theob: After Q19 R19 S18 S19, simply capturing the 4 stones and letting S18 stone be captured is incorrect. Does this mean there is a better solution ?
d4rkm4tter: In Gokyo Shumyo IV, some problems have incomplete solutions right now. There was only one correct move but I found 4 correct moves.
Andre: At the end of the sequence, taking away the last liberties of the black l-shape, it classifies P1 in stead off O1 as incorrect.
However i think that any sequence winning the liberty race is good.
Neri: O1 is better because R2 ko treat is smaller in this case.
Your move has been added.