EVENT
BOARDSIZE 19
SETUP W B16 C14 C11 L17 M16 N16 O15 P16 R16 R17 
SETUP B D16 C17 F16 J17 R18 O17 O16 N17 
MARK 1@R17
COM
Problem 123. Black to play.  

How does Black respond when White bumps into his stone with 1
? 
ENDCOM
B 1 P17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Black 1 is the vital point.
ENDCOM
  VAR
  B 1 Q18
  COM
  Failure.

  Black can live by drawing back with 1, ...
  ENDCOM
  W 2 P17
  B 3 P18
  W 4 S18
  B 5 M18
  W 6 L18
  B 7 O19
  COM
  ... but he has a smaller life and doesn't leave weaknesses in
  White's position.
  <=
  ENDCOM
  ENDVAR
W 2 S18
  VAR
  W 2 Q18
  COM
  If White 2 here, ...
  ENDCOM
  B 3 Q17
  COM
  ... Black plays 3 and he can break through White's wall.
  <=
  ENDCOM
  ENDVAR
B 3 L18
W 4 K18
B 5 M18
W 6 K17
B 7 Q18
MARK A@P15
COM
Black lives with the moves to 7. Black now aims to cut at A.
<=
ENDCOM