EVENT
BOARDSIZE 19
SETUP W E17 G16 K17 R10 R7 C4 D3 F4 C9 
SETUP B C17 D15 P16 R16 R12 Q5 Q3 L4 C5 
COM
Problem 105. Black to play.  

There are a number of big extensions Black can play. Which one
is the biggest ?? 
ENDCOM
B 1 C12
MARK B@D8 A@C13
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

If White makes a checking extension at A, the two black stones
in the upper left will come under a strong attack. Therefore,
Black extends to 1 to prevent this. The reason Black extends
so tightly is because he intends to make a shoulder hit at B
next.
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  VAR
  B 1 M17
  COM
  Failure.

  The checking extension of Black 1 is also a good move, ...
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  W 2 C13
  COM
  ... but White 2 is too big to allow; the Black stones there
  could find themselves uprooted and they would have to run for
  their lives.
  <=
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  ENDVAR
  VAR
  B 1 C11
  COM
  Failure.

  Black 1 here ...
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  W 2 E9
  MARK \t@C5 A@C13
  COM
  ... could provoke White to jump to 2 and the marked black stone
  would be swallowed up in the white moyo. After this, White could
  aim at the invasion at A.
  <=
  ENDCOM
  ENDVAR
W 2 N17
COM
White 2 and ...
ENDCOM
B 3 S6
COM
... Black 3 are the natural continuation.
<=
ENDCOM