(;
GM[1]
SZ[19]
AW[cb][dc][eb][gd][co][fo][pp]
AB[cc][cd][de][ck][dq][gq][qd]
C[Problem 166. Black to play.  

Black should be happy about his result on the left side. Can he now switch to the right side, or is there anything else still remaining to be done on the left ? ]
(;B[dh]
TR[gq][fo]

TE[2]
C[Correct Answer.  

Defending with Black 1 is the best move. This makes the exchange of the marked stones bad for White. White's two stones on the lower left are now caught between two strong black positions. <= ]
)
(;B[oc]
C[Failure.  

Making a corner enclosure with Black 1 in not good. Black's position on the left is too thin. ]
(;W[ch]
C[White will invade with 2 and ... ]
(;B[fe];W[eh];B[ek];W[gh]
C[... Black is left with two weak groups to defend after White 6. <= ]
)
(;B[eh]
LB[ch:2]
C[The cap.  

Black might try to counter White's invasion at 2 by capping with 3, but ... ]
;W[di];B[ek];W[ei]
C[... White simply moves out into the center. <= ]
))
(;W[dh]
LB[bh:A]
C[Too high.  

White 2 on the fourth line is too high. If Black's stones get into trouble, he can always link up at A. <= ]
))
)