(;AB[eq][cp][dp][hp][pp][bo][eo][fo][go][ho][cn][pn]AW[er][bq][cq][dq][hq][iq][nq][bp][fp][gp]C[Black plays.

Source:
     Tesuji Dictionary, by Segoe Kensaku & Go Seigen]SZ[19]

(;B[fq]C[A simple mistake.];W[fr]C[White has this simple response.];B[gq];W[gr]C[Black captures 2 stones, but White connects underneath.  Black can do much better.])
(;B[fr]C[Black must block at the vital point.]
(;W[dr]
(;B[gq]C[Another mistake.];W[gr];B[ep];W[fs]C[White makes simple moves to connect, even though Black captures 2 stones.])
(;B[ep]C[Correct.];W[gr];B[fs]C[Black successfully splits White into 2 weak groups.]))
(;W[gr]
(;B[fq]C[Another mistake.];W[gq];B[ep];W[fs]C[White makes obvious moves to connect all his stones.])
(;B[dr]C[Correct.];W[ep];B[es];W[gq]C[Protects against the snapback.];B[br]C[Black easily wins the semeai.]))))