SilverHawk: Why not at a? White cannot approach on either side due to shortage of liberties, and if White ever captures the marked stone, Black can play b to put White in atari.
fractic; at a captures the two white corner stones but it lets the other 4 white stones off the hook.
The following discussion is pitched at a slightly higher level than the problem, but it is still revelant to the problem.
Without the black stone at a, can extend and say arigato to Black for the free gift. Obviously White should not play at b otherwise black can play at and thank White instead.
So without Black a, Black should atari at . The correct response is , regardless of whether there is a Black stone at a. This is probably a IGS 1k* level indicator.