EVENT
BOARDSIZE 19
SETUP W D4 D16 R11 
SETUP B R16 P16 Q4 
MARK \t@R11
COM
Problem 1. Black to play.  

How should Black attack the marked white stone ? 
ENDCOM
B 1 R9
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.  

The white stone is outnumbered three to one. In such cases, it
is important for Black to press his advantage and immediately
attack. In this position Black 1 is the correct direction; this
move drives White toward Black's strong position at the top.
ENDCOM
  VAR
  B 1 R13
  COM
  Failure.  

  Black might make a checking extension from above with 1, but
  ...  
  ENDCOM
  W 2 R8
  B 3 R6
  COM
  ... this stone is inefficient because it is too close to Black's
  strong enclosure above. In other words, Blacks stones are overconcentrated.
  <=
  ENDCOM
  ENDVAR
W 2 R14
B 3 P14
COM
Next, Black caps with 3 ...  
ENDCOM
W 4 P11
B 5 P9
W 6 N11
MARK B@L17 A@Q3
COM
... the sequence to White 6 follows naturally. Black has build
a moyo on the lower right in sente. He can next reinforce his
position at the bottom with A or take up position at the top
with B.
<=
ENDCOM