EVENT
BOARDSIZE 19
SETUP W D16 D14 D10 D4 K4 R14 Q9 P9 O9 O10 R8 
SETUP B Q3 F17 K16 Q16 S15 R12 R10 R9 P10 P11 N12 
COM
Problem 170. Black to play.  

Black's moyo in the upper right has almost turned into sure territory.
However, his stone in the lower right corner seems to have gotten
left behind in the battle above. Black must not forsake this
stone, but what is the best way to strengthen it ? 
ENDCOM
B 1 N4
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Because White is thick above, Black should extend tightly with
the large knight's move of 1. Black's stones at the bottom are
now safe.
<=
ENDCOM
  VAR
  B 1 M4
  COM
  Dubious.

  Extending all the way to 1 leaves Black a bit thin.
  ENDCOM
  W 2 K6
  MARK A@O4
  COM
  White will jump to 2, and Black will be troubled by the possibility
  of a white invasion at A.
  ENDCOM
  B 3 M6
  COM
  If Black jumps to strengthen his stones, ...
  ENDCOM
  W 4 L8
  COM
  ... White will play 4, building up a moyo in the center.
  <=
  ENDCOM
  ENDVAR
  VAR
  B 1 R5
  COM
  Failure.

  Making a corner enclosure with 1 is overly defensive.
  ENDCOM
  W 2 N4
  MARK 1@R5
  COM
  White now takes the strategic point of 2. Compare this diagram
  with the Correct Answer and you will understand why Black 1 is
  not an efficient move.
  <=
  ENDCOM
  ENDVAR