EVENT
BOARDSIZE 19
SETUP W D4 D16 K17 M18 N17 O17 O15 
SETUP B M17 M15 Q16 Q14 Q10 Q4 K4 
MARK \t@N17 \t@M15
COM
Problem 110. Black to play.  

In answer to Black's marked stone, White has strengthened his
position at the top with his own marked stone. How should Black
continue ? 
ENDCOM
B 1 O14
MARK A@O13
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Attaching with Black 1 is absolutely essential so that Black
can maintain a connection with his two stones on the right. If
Black omits 1, White will jump to A. This black move also maps
out a vast moyo on the right side, ...
ENDCOM
W 2 R6
COM
... so White must invade on the lower right with 2.
ENDCOM
  VAR
  W 2 P15
  MARK 1@O14
  COM
  Variation.

  White might respond to Black 1 ...
  ENDCOM
  B 3 P14
  W 4 Q15
  B 5 R15
  W 6 R16
  COM
  ... with the sequence to 6.
  ENDCOM
  B 7 S16
  COM
  Black sacrifices a stone when he ataris at 7 and ...
  ENDCOM
  W 8 R17
  B 9 S14
  W 10 S17
  B 11 N15
  W 12 N16
  COM
  ... ends in sente with the sequence to White 12.
  <=
  ENDCOM
  ENDVAR
  VAR
  W 2 N14
  COM
  Variation.

  White might also play 2 here.
  ENDCOM
  B 3 N15
  COM
  In that case Black will cut at 3, getting the advantage in the
  fight that follows.
  <=
  ENDCOM
  ENDVAR
B 3 R5
COM
However, Black goes on the attack with 3 ...
ENDCOM
W 4 Q6
B 5 O4
COM
... and 5.
ENDCOM