EVENT
BOARDSIZE 19
SETUP W J17 N16 O17 P16 
SETUP B D16 G17 P17 Q17 R16 R14 
MARK A@J15
COM
Problem 27. Black to play.  

Black attacks with the marked stone, aiming to invade. If White
doesn't defend by playing A, where does Black invade ? 
ENDCOM
B 1 M17
MARK A@O18
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black 1 is the vital point. Black threatens to link up with his
stones on the right by playing at A.
ENDCOM
  VAR
  B 1 L17
  COM
  Failure.

  An invasion at Black 1 is also possible, but ...
  ENDCOM
  W 2 K16
  B 3 L16
  W 4 L15
  B 5 J18
  W 6 H18
  B 7 K18
  W 8 H17
  B 9 G18
  W 10 M15
  B 11 H19
  W 12 H15
  COM
  ... with the moves to 12, White gets a thick shape in the center
  of the board.
  <=
  ENDCOM
  ENDVAR
W 2 N18
COM
White must defend at 2.
<=
ENDCOM