EVENT
BOARDSIZE 19
SETUP W D16 C17 F16 L17 
SETUP B N17 Q16 R15 B16 C14 C11 
COM
Problem 53. White to play.  

Black's three stones on the right are a bit thin. Where is the
vital point to invade ? 
ENDCOM
W 1 P17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

White 1 is the vital point.
ENDCOM
B 2 Q17
COM
After Black answers with 2, ...
ENDCOM
W 3 Q18
  VAR
  W 3 P16
  MARK 2@Q17
  COM
  Failure.

  Answering 2 with 3 is bad.
  ENDCOM
  B 4 P15
  COM
  Black forces with 4, ...
  ENDCOM
  W 5 O16
  B 6 P18
  W 7 O18
  B 8 Q18
  MARK 6@P18
  COM
  ... 6, and 8.
  ENDCOM
  W 9 O17
  COM
  After 9, White is left with bad shape.
  <=
  ENDCOM
  ENDVAR
B 4 P16
W 5 O17
B 6 O16
W 7 N18
B 8 R18
W 9 P18
COM
... White will link up with the moves to 9.
ENDCOM
B 10 N16
COM
<=
ENDCOM