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Diagram 20.1: White to play
Does help in any way?

Diagram 20.2: False hope
is incorrect. Black is expecting this sequence similar to Diagram 19.2.
would help in this case. However......

Diagram 20.3: Black fails
is the correct answer.
Note that white can use this sequence in Shape 19 as well.

Diagram 20.4: Continued (12= , 13= )
Black is one liberty too short and cannot play 'a'.

Diagram 20.5: Black fails
So black has to block at .
However, is incorrect.
This result is the same as Diagram 19.3 and black fails to make use of .

Diagram 20.6: Solution 1
Unlike Diagram 19.4, tesuji now works.
Thanks to black can throw in to start a ko fight.

Diagram 20.7: Variation
may atari from the other side, forming a different ko. But note that
black takes the ko first so this result cannot be good for white.

Diagram 20.8: Solution 2
Since in the previous diagram appears to be the key point,
wants to occupy that location first.
To avoid the failure in Diagram 20.5,
is a must. is a nice idea.
Black has to go back to to avoid the Bulky Five killing shape.
So the result is a ko.
If black wins the ko fight by connecting at ,
white can still play 'a' to form a seki, destroying 9 points of territory,
although black also has a yose at 'b' that can get a few points back.
Overall this solution seems to be slightly worse than
Solution 1 for black.

Diagram 20.9: White fails
is a thoughtless move, hoping black to block at 'a' so that knowledge in Shape 7 can apply.
But is good - please verify that black is unconditionally alive now.
To summarise: black corner is quite resilient because of the hane move.
Diagram 20.6 and Diagram 20.8 are the right solutions, resulting
in ko.
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