EVENT
BOARDSIZE 19
SETUP W D18 E18 E16 F16 G16 M17 R14 R11 Q7 R6 Q5 D4 D10 
SETUP B C18 B17 D17 D16 E15 F15 G17 O17 Q16 R16 R8 R5 R4 Q3 O3 
MARK A@R8 \t@R14
COM
Problem 13. White to play.  

Black has invaded at A. How should White respond ? 

Note the presence of the marked White stone.  

[You will find a supplement to this Problem Diagram as "Problem 137"]
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W 1 S7
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

White 1 prevents Black from linking up to his stones.
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B 2 S10
COM
If Black 2, White plays on top with 3 and ...
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  VAR
  B 2 Q10
  COM
  Variation.

  Instead of trying to live on the side, Black might play on the
  outside with 2 here.
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  W 3 R10
  B 4 R9
  W 5 S9
  MARK 3@R10
  COM
  But after White 3 and 5, Black is left with three burdensome
  stones which can be easily attacked.
  <=
  ENDCOM
  ENDVAR
W 3 R10
B 4 S11
COM
... Black crawls with 4.
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W 5 R12
COM
White 5 is now the  key move, ...
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  VAR
  W 5 R9
  MARK 4@S11
  COM
  Failure.

  After Black 4 in the Correct Answer, White should not draw back
  to 5 here.
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  B 6 R12
  COM
  Black would then hane at 6 and ...
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  W 7 Q12
  B 8 R13
  W 9 Q13
  B 10 S14
  W 11 Q14
  B 12 S13
  COM
  ... link up his stones with ones above with the sequence to 12.
  Black's invasion has succeeded.
  <=
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  ENDVAR
B 6 R9
COM
... forcing Black to come back to correct the defect in his position
by playing 6, but ...
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W 7 P9
MARK \t@S7
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... White confines the black stones with the knight's move of
7. Since White 1 (the marked stone) is on the vital point, Black's
stones have no chance of making two eyes.
<=
ENDCOM