jms: I think we can safely ignore the stone. If is played at , it is gote for black (and a mistake, as the move is uneccesary).
Instead, if black plays at 'a', is still fine. Could B play at b to threaten an eye?
jms: There are many possible continuations - but I think B has no eyespace after
This variant, playing Black's best answer before he can do so, leads to something similar to a L+2-group which is a much easier shape than the carpenter's square. a and b are miai.
unkx80: I'll assume that is Black's sente. Then after , White has to play at instead of a.