(;
GM[1]
SZ[19]
AW[dc][dp][cm][cd][ce][fc]
AB[pd][pp][de][cf][dg][ck]
LB[de:1][cd:4][ce:2][cf:3][dg:5][fc:6][ck:7][cm:8]
C[Problem 21. Black to play.  

If Black plays the joseki to 7, how should he respond to White 8 ? ]
(;B[ek]
LB[ch:A]

TE[2]
C[Correct Answer.  

White is threatening to invade at A, so Black should defend his position on the left side by jumping to 1. ]
;W[qj]
LB[ql:B][qh:B]
C[White would then invade at 2. From this position, Black would play one of the points marked B and a joseki which involves the whole right side would be put in motion. (See Get Strong at Joseki 3, Joseki 9.) <= ]
)
(;B[pj]
C[Failure.  

It is tempting to make a sanren-sei on the right side with 1, but ... ]
;W[ch]
C[... in that case White would invade at 2 and ... ]
;B[bg];W[dj];B[cj];W[ci];B[bi];W[bh];B[ah];W[bj];B[ai];W[dk];B[bk];W[cl]
TR[fc]
C[... make thickness with the sequence to 14. This thickness and White's marked stone now serve to diminish the power of the sanren-sei formation on the right side. <= ]
)
)