caranthir: I can't see how w is alive after said sequence B18, A18, A19. AFter b takes with B19, it seems to me that at best w will have a ko shape.
d4rkm4tter: Difficult! Here is what I found: The sequence B18-A18-D18 which was previously marked correct is wrong. However, the sequence B18-A18-A19-B19 is indeed a living shape. You checked to expand the eye space by continuing with D18 or B15. This creates an empty triangle when you would try to capture the inside stones. You can though live with C19 or A18. This sets up a double atari and the player is alive unconditionally. The key to this problem was avoiding an empty triangle which reduces liberties.
TherapieGo: Hi, problem 21 "In-seong's Lectures" is a mistake since White can live without the double ko.
If White plays B18 and Black answers A18 White plays A19 and it is alive unconditionally.
Double ko is not 100% unconditional.
SGF