(;
AB[ib][pb][pc][pd][pe][pf][hc][jc][oc][qc][rc][id][kd][ld][nd][le][oe][qe][nf][og][qg]
AW[pa][ob][qb][rb][sb][kc][nc][md][qf][rf][pg][ph][rg][oi][oj][qi]
C[White has just descended with the marked play, which threatens to connect down the right side.  How should Black respond?

Source:
    American Go Yearbook 2004, p. 17
    Rui Naiwei 9P (black) v.  Cho Hunhyeon 9P]
TR[sb]
(;
B[nb]
C[In the game, Black correctly blocks on this side.];
W[sc]
C[White can connect here now, but Black's reduction of White's aji at the top is bigger. ]
)
    (;
    B[re]
    C[Preventing White from connecting down the right side, by descending here for example, is a mistake.];
    W[mb]
    C[Black needs to be able to quickly see that White can make a 2nd eye along the top now.];
    B[lc]
    C[For example ...];
    W[mc];B[na];W[oa];B[ma]
    C[If Black persists, ...];
    W[lb];B[la];W[ka];B[kb];W[nb]
    C[... White makes a 2nd eye, regardless.]
    )
)

