EVENT
BOARDSIZE 19
SETUP W D17 E16 P3 R4 O17 L17 Q18 
SETUP B C15 C11 C5 D3 R10 Q16 R17 Q14 
COM
Problem 163. White to play.  

The only moves left are extensions on the side. Where should
White play ? 
ENDCOM
W 1 J3
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

White should extend all the way to 1.
ENDCOM
  VAR
  W 1 J4
  COM
  Failure.

  White must not make a high extension to 1, ...
  ENDCOM
  B 2 G3
  COM
  ... because  Black 2 threatens an invasion on the lower side.
  <=
  ENDCOM
  ENDVAR
  VAR
  W 1 R8
  COM
  Failure.

  The checking extension on the ride side with 1 is not necessarily
  bad, but ...
  ENDCOM
  B 2 K3
  MARK 1@R8
  COM
  ... Black will extend to 2 and make an ideal double-wing formation
  from his corner enclosure in the lower left. For that reason,
  White 1 must be criticised.
  <=
  ENDCOM
  ENDVAR
B 2 R7
COM
If Black 2, ...
ENDCOM
  VAR
  B 2 G3
  COM
  If Black 2 here, ...
  ENDCOM
  W 3 R8
  COM
  ... White takes the last big extension at 3.
  <=
  ENDCOM
  ENDVAR
W 3 F3
COM
... White can make a checking extension with 3.
<=
ENDCOM