(;
AB[bb][cb][cc][dd][ed][ee][fd][ec][eb][cf][ci][dl][cn][fp][pd][pp][nc][bp][bq][cq][dq][ep][fq][fr][go][gn][gm][hm][im][qm][np][lq]
AW[bc][cd][ce][de][dc][ef][ff][fe][gd][fc][ch][ck][cl][dp][bh][dm][dn][cp][eo][fo][gp][gq][gr][ho][hn][jn][jp][bo][nq][qf][qj]
PL[W]
TR[np]
C[White's response to the marked attachment?

Source:
     American Go Journal, Vol. 34, No. 1, p. 15
    1999 World Amateur Championship
    Yi Guo Sun (China) vs. Ted Ning (USA) (White)
    Commentary:  Jie Li]
(;
W[jm]
C[In the game, White incorrectly pushes from behind against Black's weak center left group.];
B[jl];W[kl];B[jk];W[kk];B[jj]
TR[jm]
C[White center thickness is not effective.  These exchanges make Black's center group stronger.  But also, when White plays in the lower right corner, Black can more flexibly respond, now that the center shape is fixed.

The earlier variation to White's incorrect marked play show how White should have first probed the lower right corner to see how Black would have responded.];
W[mp]
C[The following game sequence shows how White tries in vain to make something useful out of the corner.];
B[mq];W[op];B[no];W[oq];B[oo];W[pq];B[qq];W[qp];B[po];W[qr];B[rq]
(;
W[rp]
C[A suicide move, but the variation shows that Black is still definitely leading here.];
B[rr];W[rs];B[pr];W[sq];B[qs];W[sr];B[ss];W[er]
C[Actually, this move is not a big ko threat.];
B[nr];W[eq];B[co];W[fs];B[bm]
C[This lower left exchange is not good for White, because Black's corner is alive.]
)
    (;
    W[rr]
    C[White can live, but ...];
    B[rp];W[mr];B[lr];W[ns];B[ko];W[jo];B[kj]
    C[... when Black uses sente to turn here, the result is better for Black.]
    )
)
    (;
    W[op]
    C[Correct.  White should have probed here first.]
    (;
    B[oo]
    C[If Black blocks on the outside, ...];
    W[oq];B[mp];W[pq];B[qq];W[qr];B[qp];W[rr];B[mr];W[nr]
    C[... White destroys Black's corner, and Black's outside group is not strong.]
    )
        (;
        B[oq]
        C[If Black saves the corner, ...];
        W[oo];B[pq];W[mp];B[nr];W[no];B[pn];W[kl];B[ij];W[ki]
        LB[mr:A]
        C[... White can attack the center left group more effectively, using the thickness of the lower right stones.  And the aji of A is an added bonus.]
        )
    )
)

