(;
GM[1]
SZ[19]
AW[dc][ce][dq][co][cn][po][kh][mh][ni][nh][oh][ph][pf][qf][qi][qj]
AB[dn][dl][pq][kc][nd][pd][qe][ng][qh][pi][oi][nk][pk][qk][rj][ri]
TR[po]
C[Problem 12. Black to play.  

White has invaded inside Black's sphere of influence with the marked stone. How should Black respond ? ]
(;B[pp]
TR[po]

TE[2]
C[Correct Answer.  

Bumping against the white stone with Black 1 is the strongest move. ]
;W[qo];B[rp];W[no]
C[The sequence to 4 can be expected. Next, ... ]
(;B[mp]
C[... Black stakes out the territory at the bottom with 5. ]
;W[ol]
C[White's stones are still insecure, so she has to escape into the center. She peeps with 6, then ... ]
;B[ok];W[ln]
C[... jumps to 8. ]
;B[lp]
C[Black calmly defends with 8. ]
;W[ll];B[lk];W[kl];B[li];W[lh];B[kk];W[jl]
C[With 16, White is out into the center, but ... ]
;B[ij]
C[... Black 17 keeps the white stones above and below separated. <= ]
)
(;B[mq]
LB[mp:@]
C[Caution.  

Black 5 (@) in the Correct Answer is an important move. Black must not play 5 on the third line because ... ]
;W[ol];B[ok];W[lo]
LB[lq:A]
C[... White can jump to 8 and her stones are savely out in the open. Moreover, White is threatening the attachment at A, so Black might have to defend against it. ]
))
(;B[np]
C[Failure.  

The knight's move of Black 1 is the standard response to the white approach move, but ... ]
;W[qq];B[qr];W[no];B[mo];W[pr]
C[... White can make sabaki with the sequence to 6. <= ]
)
(;B[qp]
C[Failure.  

If Black 1 here, ... ]
;W[mo]
C[... White will jump to 2. Either way, Black's thickness above has been neutralised. <= ]
)
)