EVENT
BOARDSIZE 19
SETUP W D16 O3 L3 L5 E3 D4 D6 
SETUP B Q16 P5 R5 Q3 J3 F3 F4 
MARK A@Q11
COM
Supplement to Problem 23: Correct for Black.

Instead of playing at A in the Problem Diagram, ...
ENDCOM
B 1 J5
USER
CORRECT ANSWER
ENDUSER
COM
... Black should have reinforced his stones at the bottom with
1.
ENDCOM
W 2 N5
COM
Now it is White who must defend with 2. Next, ...
ENDCOM
  VAR
  W 2 C9
  MARK @@N5
  COM
  If White omits @ and extends to 2, ...
  ENDCOM
  B 3 M4
  COM
  ... Black will attack White's stones at 3 ...
  ENDCOM
  W 4 L4
  B 5 N2
  COM
  ... and 5.
  <=
  ENDCOM
  ENDVAR
B 3 Q11
COM
... Black can extend to 3.
<=
ENDCOM