EVENT
BOARDSIZE 19
SETUP W C10 C7 D4 D3 L3 O3 Q4 Q16 P16 R17 S17 
SETUP B D17 C14 F3 J3 J5 R6 Q9 L17 R16 Q15 R14 S15 
MARK \t@P16 \t@Q16 \t@R17 \t@S17
COM
Problem 26. Black to play.  

The four marked stones seem to be secure, but Black has a way
to uproot them. How should Black attack ? 
ENDCOM
B 1 P18
MARK A@Q17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black 1 is the vital point. It aims to cut at A. This is a typical
example where it is strategically best to gouge out the opponent's
base.
ENDCOM
  VAR
  B 1 O17
  COM
  Failure.

  Black 1 is a slack move.
  ENDCOM
  W 2 P18
  B 3 P17
  W 4 Q17
  B 5 O18
  W 6 Q19
  COM
  White easily makes life in the corner with the sequence to 6.
  <=
  ENDCOM
  ENDVAR
W 2 Q18
COM
White must defend with 2, but ...
ENDCOM
  VAR
  W 2 O17
  MARK \t@P18
  COM
  Variation.

  Attacking the marked stone with 2 ...
  ENDCOM
  B 3 O18
  W 4 N17
  COM
  ... and 4 doesn't accomplish much.
  ENDCOM
  B 5 N18
  MARK 3@O18
  COM
  Black links up his stones with 3 and 5, so ...
  ENDCOM
  W 6 Q17
  COM
  ... White must come back to defend the cut with 5.
  ENDCOM
  B 7 N15
  COM
  Black can now attack at the vital point of 7.
  <=
  ENDCOM
  ENDVAR
  VAR
  W 2 P17
  COM
  Variation.

  White 2 ...
  ENDCOM
  B 3 O18
  W 4 Q18
  COM
  ... and 4 is another way for White to defend, but ...
  ENDCOM
  B 5 N16
  MARK 3@O18
  COM
  ... her stones are still under attack after Black 3 and 5.
  <=
  ENDCOM
  ENDVAR
B 3 P17
W 4 Q17
B 5 N17
COM
... Black makes shape at the top with the sequence to 4. The
white stones are left without two eyes, so White must escape
into the center. Black can make profit by threatening these stones.
<=
ENDCOM