(;AB[bq][eq][pq][bp][cp][ep][op][do][eo][en][fn]AW[cq][dq][hq][kq][dp][co][qo][cn][dn][em][cj]C[White plays in the lower left corner.

 Source:
 Tesuji Dictionary, by Segoe Kensaku & Go Seigen]SZ[19]
;W[br]C[OK, the 1st move is easy.];B[ar]
(;W[cr]C[Slack.];B[bs];W[cs];B[ds]LB[aq:A]C[White has given Black a better ko.  Black has the option of a local ko threat at A.  In addition, if White wins the ko, Black can tenuki, because White cannot connect up with the 2 lower middle stones.])
(;W[bs]C[Correct way to reach the ko.];B[dr];W[cr];B[es];W[bo];B[cs];W[ds]TR[dr]C[White takes the ko first, and Black has no local ko threats.  And now if Black tenukis, White captures the marked stone, threatening to reduce Black's pressure on the 2 middle bottom stones.]))