(;
GM[1]
SZ[19]
AW[ep][fq][dc][cf][oc][lp][mp][np][op][qo]
AB[cp][dq][eq][fp][qd][pe][mq][nq][pq][oq]
TR[lp]
C[Problem 98. Black to play.  

White has just extended with the marked stone, making a thick wall at the bottom. How should Black answer this move ? ]
(;B[qi]
TE[2]
C[Correct Answer.  

Black must extend down the right side to neutralise White's thickness, but because White is so thick below Black should extend only as far as 1. ]
(;W[pg]
C[Next, if White 2, ... ]
;B[qg]
C[... Black can defend at 3. <= ]
)
(;W[qg]
C[If White 2, ... ]
;B[pg]
C[... Black plays 3. <= ]
))
(;B[qj]
C[Failure.  

Extending one line farther with 1 leaves Black vulnerable ... ]
;W[pg]
LB[qh:A]
C[... to an invasion at 2 or A. White may not make this invasion immediately, but Black must always worry about it. ]
;B[qm]
C[Black could settle his stones with 3, but White will use her thickness below to attack. <= ]
)
)