(;AB[df][ce][oc]AW[dc][dh][gd][qd]TR[gd]C[Black's response in the upper left to the marked stone?

Source:
    "Erasing Potential", by Honda Kunihisa 9p
    Go World #7, p. 48]SZ[19];LB[cc:A][bc:B]C[Hint:  Black A or B?]
(;B[cc]C[The contact play here is a bad move that erases valuable aji.];W[cb]C[The following sequence can be expected.];B[dd];W[ec];B[bc];W[kc]C[All of White's stones along the top are working efficiently.

In addition, Black has forced White to fix up a White weakness that the next variation illustrates.])
(;B[bc]C[Instead, simply sliding into the corner here is correct.];W[kc]C[Now, if White makes the desired extension toward the upper right, ...];B[fc]C[... Black can expose White's weakness by invading.]
(;W[fd]C[Blocking on the top is the usual response.];B[ec];W[ed];B[db]C[After Black crosses under, White has a couple possible responses.]
(;W[gc]C[If White blocks on the right, ...];B[dd]C[... Black simply cuts here to take the corner.])
(;W[dd]C[If White connects on the left, ...];B[cb]C[... Black simply pulls back.];W[gc]C[White cannot omit blocking on the right, so White is not happy to end in gote.]))
(;W[gc]C[White does have the option of the descending block.  The following moves form a set pattern.];B[fd]
(;W[gb];B[fb];W[fe];B[ed]C[White is satisfied to force Black to make an empty triangle, but Black has nice corner territory.])
(;W[fb]C[White can hane, but ...];B[eb];W[gb];B[ec];W[fe];B[ee]C[... Black has a similar good result.]))))