(;
GM[1]
SZ[19]
AW[cd][ec][mc][me][mg][pj][pi][qj][pk][mq][oq][pq][bp][bq][cm][cj]
AB[hc][kc][ke][kg][pc][qe][qg][oj][qk][ql][qo][op][np][dp][cq][gq]
C[Problem 15. Black to play.  

The white stones in the upper right are still eyeless. How should Black attack them ? ]
;B[nf]
TE[2]
C[Correct Answer.  

Black first peeps with 1, then ... ]
;W[mf];B[oh]
C[... makes a shoulder hit with 3 to split White into two groups. ]
(;W[oi]
C[If White 4, ... ]
;B[nh]
C[... Black extends to 5, leaving White with two isolated and eyeless groups. If White tries to save one of then, the other will fall. This is a good example of a double attack. <= ]
)
(;W[mi]
LB[oh:3]
C[Variation.  

In answer to Black 3, White might immediately jump out to 4. ]
;B[oi];W[pl];B[qm];W[nl]
C[The sequence continues to White 8, but ... ]
;B[mj]
LB[mp:B][nl:8][nk:A]
C[... Black leans on the white group at the top by attaching with 9. If White 8 at A, Black will still attach at 9. If Black needs more strength to attack the white group on the right, Black can also lean on the white group at the bottom with the forcing move at B. <= ]
)
)