EVENT
BOARDSIZE 19
SETUP W D16 C13 C5 C4 D3 F3 R10 
SETUP B C11 D7 C6 D5 Q4 R16 P17 
MARK \t@C13
COM
Problem 60. Black to play.  

The marked white stone threatens to invade the black formation
on the left side. Should Black defend against this threat or
is there something else he should do ? 
ENDCOM
B 1 R12
MARK \t@C13
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Instead of defending against the marked stone, Black should start
operations against the white stone on the right by extending
from his corner enclosure above with 1.
ENDCOM
W 2 R7
B 3 N3
  VAR
  B 3 K17
  COM
  Failure.

  Black must not rush to play the big point at the top with 3.
  ENDCOM
  W 4 O3
  MARK \t@Q4
  COM
  White will take the initiative at the bottom by playing a double
  approach against the marked black stone with 4.
  <=
  ENDCOM
  ENDVAR
W 4 S4
COM
The sequence to 4 is the best continuation.
<=
ENDCOM
  VAR
  W 4 K17
  COM
  If White 4 here, ...
  ENDCOM
  B 5 R5
  COM
  ... Black 5 takes the corner territory while attacking the white
  stones above.
  <=
  ENDCOM
  ENDVAR