EVENT
BOARDSIZE 19
SETUP W D18 F17 H16 K16 N17 N15 P15 
SETUP B C17 D16 D14 C10 P17 R16 Q14 Q12 
MARK 2@Q14 1@P15
COM
Problem 99. White to play.  

After the exchange of White 1 for Black 2, Black's position looks
very strong. However, there is a way for White to launch a successful
invasion.  
ENDCOM
W 1 R17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

White 1 is a tesuji.
ENDCOM
B 2 S17
COM
If Black blocks at 2, ...
ENDCOM
  VAR
  B 2 Q16
  COM
  Variation.

  If Black plays 2, ...
  ENDCOM
  W 3 Q18
  B 4 P18
  W 5 S16
  B 6 S15
  W 7 S18
  B 8 T16
  W 9 T17
  COM
  ... White can live in the corner with a ko by playing the moves
  to 9.
  ENDCOM
  B 10 S17
  COM
  <=
  ENDCOM
  ENDVAR
W 3 S16
COM
... White crosscuts with 3. Next, ...
ENDCOM
B 4 R18
COM
... if Black 4, ...
ENDCOM
  VAR
  B 4 S15
  COM
  ... if Black 4, ...
  ENDCOM
  W 5 Q16
  COM
  ... White 5. White has succeeded.
  <=
  ENDCOM
  ENDVAR
W 5 R15
COM
... White 5.
<=
ENDCOM