EVENT
BOARDSIZE 19
SETUP W C17 E16 K17 R11 P11 Q5 R6 Q7 F4 C3 C8 
SETUP B M17 P17 R16 R13 R5 R4 Q3 O3 C10 C15 D13 
MARK \t@F4 A@K15
COM
Problem 69. Black to play.  

White has just played the marked stone, but playing at A would
have been better What should Black do ? 
ENDCOM
B 1 G17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black 1 is the point to invade.
ENDCOM
W 2 F17
COM
White has to defend her corner with 2.
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B 3 G16
COM
After Black 3, ...
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W 4 E14
COM
... White defends with 4, but this move has little effect on
the black stones on the left.
ENDCOM
B 5 K15
COM
Black now caps with 5.
ENDCOM
W 6 L16
COM
If White runs away with 6, ...
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B 7 M14
COM
... Black traps the white stone with 7.
ENDCOM
W 8 N17
COM
White 8 is a tesuji-like move, but, ...
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B 9 N18
COM
if Black calmly links up underneath with 9, ...
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  VAR
  B 9 N16
  MARK 8@N17
  COM
  Failure.

  If Black aggressively counters White 8 by blocking on the outside
  with 9, ...
  ENDCOM
  W 10 M16
  B 11 N18
  W 12 N15
  B 13 O17
  W 14 L14
  MARK 13@O17
  COM
  ... White 14 becomes an effective tesuji after Black captures
  with 13. Black can't keep White confined.
  <=
  ENDCOM
  ENDVAR
W 10 M16
B 11 L18
COM
... then defends with 11, White's stones are still trapped.
<=
ENDCOM