All you need to know about Carpenter's Square - 28
Can the external liberty make any difference?
The external liberty is clearly helpful. If white applies the solution of Shape 27, black is able to play so the corner is alive. White has to change strategy.
is the key point. However, is incorrect. is again a calm move. Black is alive. If mistakenly plays at first, then a ko fight in inevitable.
doesn't work either. is good. Please verify the corner is alive.
is a brilliant move, forming a ko fight easily.
is useless. Black can only create a different ko fight, which is favourable for white because white takes the ko first.
Conclusion: Diagram 28.5 is the best for both sides.