EVENT
BOARDSIZE 19
SETUP W C16 E16 K17 N17 R14 Q14 R10 P10 
SETUP B P17 Q16 R15 C10 
COM
Problem 129. White to play.  

The shape of the black stones is called 'the three crows'. This
is a very solid formation, but is still vulnerable to an invasion
if White has built up strong positions in the vicinity. Where
does White invade ? 
ENDCOM
W 1 R17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

White 1 is the vital point.
ENDCOM
B 2 R16
COM
Black 2 is the severest response.
ENDCOM
  VAR
  B 2 S16
  COM
  Variation.

  If Black 2 here, ...
  ENDCOM
  W 3 S17
  B 4 S14
  W 5 P18
  B 6 O18
  W 7 Q18
  B 8 O17
  W 9 N18
  B 10 O19
  W 11 S18
  COM
  ... White easily lives with the moves to 11.
  <=
  ENDCOM
  ENDVAR
W 3 Q18
B 4 P18
W 5 S18
B 6 S16
W 7 R19
B 8 S17
W 9 Q17
B 10 T18
COM
After 10, ...
ENDCOM
W 11 P16
MARK A@S19
COM
... White cuts at 11, and the life or death of the stones in
the corner depends on a ko starting at A.
<=
ENDCOM