(;
GM[1]
SZ[19]
AW[pm][nj][nh][og][qf]
AB[jp][jd][pj][dj][dp][pd][pp][dd][jj][nq][oh][ng][nd]
LB[qf:1][nd:2][pm:3][nq:4][nj:5][oh:6][nh:7][ng:8][og:9]
C[Problem 13. Black to play.  

White cuts with 9. How should Black respond ? ]
(;B[ph]
TE[2]
C[Correct Answer.  

The proverb advises, 'When caught in a crosscut, extend !'. Black should follow this maxim by extending to 1 and strengthening his stones toward the edge. This gives him two ways to save his stones on the side. ]
(;W[mh]
C[If White extends to 2, ... ]
;B[pg]
C[... Black ataris with 4. ]
;W[qc]
C[It is unreasonable to try and save the stone in atari, so White might jump to the corner with 4. ]
;B[qd]
C[Black will block with 5 and ... ]
;W[rd];B[re];W[rc];B[qe];W[ob];B[nb];W[oc];B[nc];W[od];B[oe]
C[... confine White to the corner with the sequence to 15. ]
;W[pc]
C[White ends in gote, so ... ]
;B[ok]
C[... Black can separate White's stones in the center from the one on the side with 17. For this reason, White won't invade at 4 immediately. Instead, she will leave it in reserve as aji and strengthen her stones in the center first. <= ]
)
(;W[of]
C[Variation.  

If White extends to 2 first, ... ]
;B[ni]
C[... Black ataris from the other side with 3 and ... ]
;W[mh];B[mi]
C[... pushes out into the center with 5, building a wall and influence towards the bottom. ]
;W[lh];B[nf]
C[Black 7 now leaves White with two weak groups to defend. This result is bad for White. <= ]
))
(;B[ni]
C[How the game continued.  

Black 1 ... ]
;W[mh];B[mi]
C[... and 3 were unrelenting, but ... ]
;W[oi]
C[... they gave White the chance to counterattack with 4 ... ]
;B[ph];W[pi]
C[... and 6. In the fight to 26, Black made many mistakes - ... ]
;B[pg]
LB[qi:A]
C[... - 7 should be at A - ... ]
;W[qi];B[pf];W[mg];B[nf]
LB[li:B]
C[... - 11 should be at B - ... ]
;W[li];B[mj];W[mk];B[lj];W[kj];B[lk];W[kk];B[ll];W[kh];B[ji];W[ki];B[nl]
LB[oj:C]
C[... - 23 should be at C - ... ]
;W[oj];B[nk];W[ok]
C[... - so White got profit on the right and established a position in the center. This sequence was a success for White. <= ]
)
)