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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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