(;
GM[1]
SZ[19]
AW[db][eb][ed][fd][gd][lc][qf][qi][pm][qn][po][dp][dj]
AB[cb][bc][dc][dd][ee][fe][gc][nc][pd][qd][ql][qo][qp][pq][nq]
LB[ql:A]
TR[qf]
C[Problem 13. White to play.  

Black has invaded at A. How should White respond ?  

Note the presence of the marked White stone.  

[You will find a supplement to this Problem Diagram as "Problem 137"] ]
;W[rm]
TE[2]
C[Correct Answer.  

White 1 prevents Black from linking up to his stones. ]
(;B[rj]
C[If Black 2, White plays on top with 3 and ... ]
;W[qj];B[ri]
C[... Black crawls with 4. ]
(;W[qh]
C[White 5 is now the  key move, ... ]
;B[qk]
C[... forcing Black to come back to correct the defect in his position by playing 6, but ... ]
;W[ok]
TR[rm]
C[... White confines the black stones with the knight's move of 7. Since White 1 (the marked stone) is on the vital point, Black's stones have no chance of making two eyes. <= ]
)
(;W[qk]
LB[ri:4]
C[Failure.  

After Black 4 in the Correct Answer, White should not draw back to 5 here. ]
;B[qh]
C[Black would then hane at 6 and ... ]
;W[ph];B[qg];W[pg];B[rf];W[pf];B[rg]
C[... link up his stones with ones above with the sequence to 12. Black's invasion has succeeded. <= ]
))
(;B[pj]
C[Variation.  

Instead of trying to live on the side, Black might play on the outside with 2 here. ]
;W[qj];B[qk];W[rk]
LB[qj:3]
C[But after White 3 and 5, Black is left with three burdensome stones which can be easily attacked. <= ]
)
)