EVENT
BOARDSIZE 19
SETUP W C15 C16 D17 F17 D4 F3 R10 R14 S16 
SETUP B D15 C14 D14 D10 C6 Q4 R8 Q16 O17 
MARK \t@S16
COM
Problem 117. Black to play.  

White has just played the marked stone. What should Black do
? 
ENDCOM
B 1 R12
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black is strong at the top, this is his chance to invade at 1.
Black's aim is to attack the lone white stone on the right.
ENDCOM
  VAR
  B 1 R17
  COM
  Failure.

  Defending the corner with Black 1 is a passive move; Black has
  failed to disrupt White's position and to take the initiative.
  ENDCOM
  W 2 Q12
  MARK A@R6
  COM
  White secures her stones with 2. Next, she aims at an invasion
  at A.
  <=
  ENDCOM
  ENDVAR
W 2 P14
COM
If White jumps to 2, 
ENDCOM
B 3 Q11
MARK \t@R10
COM
... Black plays the diagonal move of 3. The marked stone is now
under attack.
ENDCOM
W 4 R17
COM
If White 4, Black simply takes up position at the top with 5.
ENDCOM
B 5 L17
W 6 Q10
MARK \t@L17
COM
After Black played the marked stone, White will run away with
6 ...
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B 7 O11
W 8 Q8
B 9 R7
W 10 P10
COM
... to 10.
ENDCOM
B 11 O13
COM
Black makes shape with 11.
ENDCOM
W 12 N9
COM
White 12 is necessary, so ...
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B 13 O14
MARK 4@R7
COM
... Black can make a wall with 13. Although this result is not
decisive for Black, he has managed to strengthen his position
with 4 and White's stones still lack eyes.
<=
ENDCOM