(;
GM[1]
SZ[19]
AW[pd][qg][lc]
AB[dd][gc][pj]
C[Problem 144. Black to play.  

The white formation is called 'kannon-biraki' in Japanese, meaning double-hinged doors. Such formations are defective and need an additional move to complete the territory they map out. Where should Black invade ? ]
(;B[qc]
TE[2]
C[Correct Answer.  

When faced with this formation, invading at the 3-3 point and living in the corner with Black 1 is the biggest move. ]
;W[pc]
C[Of course, White blocks from the wider side with 2. ]
;B[qd];W[qe];B[re];W[rf]
LB[qe:4]
C[White 4 and 6 are the strongest moves, but ... ]
;B[pe];W[qf];B[pb];W[ob];B[rb];W[pa];B[qb];W[nc];B[rd]
C[... Black gets six points of territory after he connects with 15.  <= ]
)
(;B[pc]
C[Failure.  

Attaching with 1 is not good. ]
;W[qc];B[oc];W[qb]
LB[ic:A][oc:3][pc:1][qc:2]
C[White takes the corner with 2 and 4, leaving Black's stones 1 and 3 heavy. White can still settle her stone at the top by extending to A. <= ]
)
)