EVENT
BOARDSIZE 19
SETUP W C12 C16 E16 H17 R17 R14 Q14 R11 
SETUP B C10 K17 O16 Q16 R15 S15 
MARK 1@S15 \t@R17
COM
Problem 125. White to play.  

Black 1 is a strong response to the invasion by the marked stone.
Black intends to give White the corner and attack the right side.
What is White's next move ? 
ENDCOM
W 1 P17
MARK A@P18
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Peeping at White 1 is a tesuji. As you will see in the Failure
Diagram, White 1 at A is not good.
<=
ENDCOM
  VAR
  W 1 P18
  COM
  Failure.
  ENDCOM
  B 2 P17
  MARK 1@P18
  COM
  Black 2 is a good response to White 1.
  ENDCOM
  W 3 O18
  COM
  If White 3, ...
  ENDCOM
    VAR
    W 3 Q18
    COM
    If White draws back to 3, ...
    ENDCOM
    B 4 O18
    COM
    ... Black blocks at 4.
    ENDCOM
    W 5 S18
    COM
    White gets a small life. White's result is bad.
    <=
    ENDCOM
    ENDVAR
  B 4 Q18
  COM
  ... Black plays 4 and takes the corner.
  <=
  ENDCOM
  ENDVAR