Carpenter’s square is described on Sensei’s library.
There’s a main page which illustrates the first case (white square above).
There’s also a page about “wrong attack” which discusses the second case (black square above).
Neither of them talks about the move that firstly came to mind to me, though!
Here it is
In the second case, what if next white move is A18?
If black blocks at A16, white should win the capture race “one eye vs no eye”.
But I can’t find another good move for black…
Actually, for me it’s very hard to believe that carpenter’s square can’t be killed… it looks so weak!
I see four corners for white. Black stones on 1-1 can’t escape. So I’d say that white has a strong position and is about already alive.
Black has to live in the middle. It’s a sort of “shape game” where black can try and split white, so probably he can manage to live, but not very easily.
I think the idea is to deny white 2 eyes in each corner because of the black stones. But w has 2, even 3 directions to get a second eye for each corner. Black will maybe live too, still some space for it, but will have hard time to get more as white.
