(;
AB[qd][oc][qm][pm][om][qo][pq][fq][fp][hp][go][eo][cn]
AW[dd][dj][dp][ep][cq][dn][fn][fo][hq][po][on][nm][nl]
C[Black to play in the lower left quadrant.

Dan level players reached this position playing nothing but good moves.

Source:
     "Do-It-Yourself Commentary", Ishii Shinzo 9p
     Go World #10, p. 54]
(;
B[em]
C[The game line, but a doubtful move.  This move is forcing, but loses useful potential that the variations detail.];
W[en]
(;
B[co]
C[The game line continues with Black making another mistake here.];
W[cm]
C[White could have punished Black's mistake by ignoring the peep.  The following sequence is one possibility.];
B[bm];W[cl];B[bl];W[bk];B[bq];W[br];B[bp];W[do];B[an]
LB[iq:A]
C[Black can live, but White's outside thickness is too big.  Even worse, White A is now troublesome for Black.]
)
    (;
    B[cl]
    LB[co:A]
    C[Correct.  Black can make better shape here.  Black can peep at A later.]
    )
)
    (;
    B[cl]
    C[Instead of making forcing moves that strengthen White, Black should extend here.];
    W[dm];B[bj]
    TR[eo]
    C[The result is good for Black.  Black's left side group has good shape, and the marked stone still retains useful aji.]
    )
    (;
    B[gn]
    C[Note that in the game line, Black loses the possibility of this nice sente push, which indirectly helps protect the bottom area, and also prevents a powerful White push on this same point.];
    W[fm]
    C[White must respond.]
    )
    (;
    W[gn]
    C[For example, without Black's push on this point, White has the following sequence.]
    (;
    B[iq]
    C[Black can protect the bottom, but ...];
    W[ho];B[gp];W[io]
    LB[jq:A]
    C[White threatens to devastate Black's position in sente.  White threatens A next.]
    )
        (;
        B[hn]
        C[Blocking here is no better.];
        W[ip];B[iq];W[gq];B[gp];W[fr]
        C[White tunnels under Black's position.]
        )
    )
)

