(;
GM[1]
SZ[19]
AW[cf][dd][jc][rh][ri][qi][qj][rk][sk][sl][rm][pm][nn][qn][po][ro][sp][iq][dp][fp][cq]
AB[oc][qd][qg][rg][qh][rj][qk][ql][rl][pi][pj][ol][qo][qp][rp][rr][pq][nq][bp][cn][ck]
C[Problem 159. Black to play.  

The fuseki has ended and the middle game has begun. Black has made a thick wall on the right. What is the best way for him to utilise this wall ? ]
(;B[hc]
TE[2]
C[Correct Answer.  

Invading at 1 is the best way for Black to use his thickness. ]
;W[je]
C[If White 2 ... ]
;B[he];W[jg]
C[... and 4, ... ]
;B[hg]
LB[he:3]
C[... Black keeps up the pressure with 3 and 5. ]
;W[mc]
C[White makes a base for her stones with 6, but ... ]
;B[db]
C[... Black slides to 7 and White is behind in territory. <= ]
)
(;B[lc]
C[Failure.  

Thickness should be used to attack. If Black tries to make territory with 1 ... ]
;W[je];B[le]
C[... and 3, ... ]
;W[ec]
LB[je:2]
C[... White takes the lead with 2 and 4. The black stones on the right are now overconcentrated, while White's stones are taking territory efficiently. <= ]
)
)