EVENT
BOARDSIZE 19
SETUP W D18 F17 H17 K17 R14 Q14 R10 
SETUP B C17 D16 D14 M17 O16 Q16 R15 
COM
Problem 143. White to play.  

Black's position here is much tighter. Where should White invade
? 
ENDCOM
W 1 O18
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Invading with 1 is a good move.
ENDCOM
  VAR
  W 1 R17
  USER
  CORRECT ANSWER
  ENDUSER
  COM
  Correct Answer.

  A direct invasion here is also possible.
  <=
  ENDCOM
  ENDVAR
B 2 Q18
COM
If Black defends the corner with 2, ...
ENDCOM
  VAR
  B 2 N17
  COM
  Variation.

  If Black prevents White from linking up on the left by playing
  2, ...
  ENDCOM
  W 3 R17
  MARK 2@N17 1@O18
  COM
  ... White plays on the 3-3 point with 3. With the exchange of
  1 for 2 in place, this is an effective move.
  <=
  ENDCOM
  ENDVAR
W 3 N17
B 4 N16
W 5 M18
MARK 3@N17
COM
... White links up with 3 and 5.
<=
ENDCOM