(;
AB[ba][cb][db][bd][ae][ce][cf][fc][bi][bj][cj][ic][kc][kb]
AW[eb][dc][ed][fd][gc][hc][he][id][jd][hg][hi][ii][gj][dj][di][ci][ch][cg][bh][bf][ag][be][ck][kd]
TR[ba]
PL[W]
C[White's response to the marked play?

Source:
    28th Kisei Title game
    Hane Naoki 9P vs. Yamashita Keigo 9P]
(;
W[cc]
C[In the game, White pushed here, which may seem somewhat surprising, because ...];
B[bc];W[cd];B[da]
C[... Black lives.  Although White does close off the not so small center left area in sente.

The real issue is White's alternative in the variation.]
)
    (;
    W[ab]
    C[Many of you may have wondered why White didn't attack Black's vital point here.];
    B[da]
    C[Now White can turn the corner into a ko.  But White probably didn't want to risk losing the ko with Black also destroying White's center left territory.

But the question for this problem is what is White's best move now?]
    (;
    W[bc]
    C[If you're like me, the instinctive reaction is to incorrectly start the ko with this forcing move.];
    B[cc];W[ad]
    C[Now White has a ko, but unfortunately, Black takes 1st.]
    )
        (;
        W[ad]
        C[Correct.  Congratulations, if you spotted this throw-in.];
        B[ac];W[bc]
        C[Now White can play here to start the ko, _and_ have the advantage of taking 1st.];
        B[cc]
        C[But as mentioned earlier, this ko is risky for White.  Black has local ko threats and could ruin White's center territory.]
        )
    )
)

