EVENT
BOARDSIZE 19
SETUP W H17 M16 R16 Q14 R10 
SETUP B D17 C15 O16 P17 R18 
COM
Problem 46. Black to play.  

White's stones at the top are nicely balanced, but she is still
vulnerable to an invasion. Where should Black invade ? 
ENDCOM
B 1 K17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black 1, in the middle of the three-space extension, is the vital
point.
ENDCOM
W 2 K16
COM
White 2 is the best response, but ...
ENDCOM
B 3 L17
W 4 L16
B 5 M17
W 6 J17
B 7 N16
COM
... Black links up with the moves to 7.
<=
ENDCOM