All you need to know about Carpenter's Square - 25
The first-line hane also makes a lot of difference. What's the best strategy for white?
First of all, white cannot use the easy option as in Shape 24. This is because thanks to black has the leisure to play key point . can also play at directly. In both cases the corner is unconditionally alive.
doesn't work either, resulting in a seki. If plays instead, it is also a seki.
It may appear that tesuji and the sequence in Diagram 23.9 still work in this shape. But here is incorrect.
is a tesuji we've seen before (e.g. Diagram 5.2). Because of , is now possible.
wants to start a ko. But even if black cannot win the ko fight, there is still sufficient room to make two eyes.
here would result in a 'connect-and-die' situation and black is unconditionally alive again.
is the last option for white. This move always works in Shape 23, but we did not discuss it in details because the outcome would be inferior to the solution. Clearly is the key point. After / exchange, white has three options. Note that cannot play at 'a'.
here ends up in an approach ko favourable for black - white needs an extra move at 'a' to make it a directly ko, which is much much worse for white in Shape 23 but acceptable in this shape.
is also possible and is a tesuji. After this again results in an approach ko favourable to black. Note that if black doesn't start the ko at right away, white has the option to make the corner a seki by playing .
here is another option. Later white has the option to make a 10000-year ko at 'a' or seki at 'b'.
Conclusion: black corner is quite safe. White can choose to fight an approach ko or a 10000-year ko (which implies a seki). The situation is quite subtle and the outcome is dependent on positions elsewhere.