(;
GM[1]
SZ[19]
AW[dd][dp][kq][nq][qj][nc][md][mf]
AB[cj][pp][pn][pd][pf][oc][kc][ke]
TR[mf]
C[Problem 10. Black to play.  

White has just jumped out into the center with the marked stone. How should Black play ? ]
(;B[pi]
TE[2]
C[Correct Answer.  

Black 1 is the key point. It keeps the white stones at the top separated from their ally on the right side. ]
(;W[pj]
C[White first exchanges 2 for ... ]
;B[oi]
C[... 3, then ... ]
;W[lg]
C[... returns to take care of her stones at the top by moving out with 4. ]
;B[jg]
C[Black attacks with 5, then ... ]
;W[li];B[oj]
C[... plays another move against White's stones on the right. It is hard to predict how this fight will continue, but the white stones on the right are being severely pressed and if White tries to save then, it could spill over and affect the five white stones at the top. <= ]
)
(;W[lg]
LB[pj:@]
C[Variation.  

Instead of @ in the Correct Answer, White might immediately play the diagonal move of 2. ]
;B[jg]
C[Black could first exchange 3 for ... ]
;W[li]
C[... 4, then ... ]
;B[pj]
TR[qj]
C[... quell the marked white stone with 5. Although this stone has some aji, Black can look forward to some profit on the right side. <= ]
))
(;B[lg]
C[Failure.  

In some positions ... ]
;W[mg];B[lh];W[mh];B[li]
C[... a wall with the sequence to 5 can be good strategy, but ... ]
;W[nj]
C[... here these moves enable White to link up her stones and isolate Black's stones in the upper right. Although they are strong enough to live, White will be able to make profit by threatening them. <= ]
)
)