(;
GM[1]
SZ[19]
AW[dd][nq][kq][ko][eq][dp][dn]
AB[pd][oo][qo][pq][iq][fq][fp]
LB[pi:A]
C[Supplement to Problem 23: Correct for Black.  

Instead of playing at A in the Problem Diagram, ... ]
;B[io]
TE[2]
C[... Black should have reinforced his stones at the bottom with 1. ]
(;W[mo]
C[Now it is White who must defend with 2. Next, ... ]
;B[pi]
C[... Black can extend to 3. <= ]
)
(;W[ck]
LB[mo:@]
C[If White omits @ and extends to 2, ... ]
;B[lp]
C[... Black will attack White's stones at 3 ... ]
;W[kp];B[mr]
C[... and 5. <= ]
)
)