EVENT
BOARDSIZE 19
SETUP W D16 K16 O17 Q5 D4 
SETUP B Q16 Q14 Q9 Q3 K4 
MARK 1@Q5
COM
Problem 11. Black to play.  

White has played in the middle of Black's moyo with an approach
move at 1. How should Black respond ? 
ENDCOM
B 1 O4
MARK \t@Q9
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black should take the territory at the bottom with the knight's
move of 1. This move, in combination with the marked stone, strongly
attacks the solitary white stone on the lower right side.
<=
ENDCOM
  VAR
  B 1 P5
  MARK B@R4 A@R5
  COM
  Failure.

  There is a proverb which advises, 'Don't attach against a weak
  stone.' Black 1 violates this proverb.

  As you will later see, Black A and Black B are also bad.
  ENDCOM
  W 2 P6
  B 3 P4
  W 4 Q7
  COM
  Now the white stones, which were previously weak, become strong
  and Black has no good way to attack them.
  <=
  ENDCOM
  ENDVAR