(;
GM[1]
SZ[19]
AW[ce][dd][fc][ch][cl][bn][co][dp][fp][iq][ql][nc][nb][pc][qc][rc]
AB[cj][dl][bl][bm][cn][dn][kq][op][qp][qj][pd][qd][rd][nd][od][lc]
TR[ql]
C[Problem 42. Black to play.  

How should Black attack the marked stone ? ]
(;B[ok]
TE[2]
C[Correct Answer.  

The knight's move of Black 1 is a strong attacking move. ]
;W[qo]
C[White makes some breathing room for her stones with 2 ... ]
;B[po];W[qn]
C[... and 4, but ... ]
;B[rp]
LB[po:3]
C[... this just induces Black to secure the corner with 3 and 5. ]
;W[om]
C[When White jumps to 6, ... ]
;B[ml]
C[... Black keeps up the attack with 7, and a moyo in the top right suddenly appears that extends well into the center. The important point of this problem is that by attacking a weak white stone, Black has been able to build his moyo and make his territory at the bottom secure, while White busily tries to rescue her stones. <= ]
)
(;B[qn]
C[Failure.  

If Black plays a checking extension to 1, ... ]
;W[ol]
LB[mq:A]
C[... White can easily escape into the center with 2. Later, White can aim to invade at A. <= ]
)
)