Yesterday I happened to browse "Life and Death" by James Davies, one of the few books in the Elementary Go Series that I had missed as a beginner. This particularly nice problem caught my attention, and I could not find it on this site, so here you go.
The actual setup of the outside stones may have been different in the book, but the idea is the same.
(;AB[cr]AB[bq]AB[cp]AB[bo]AW[cq]AW[dq]AW[eq]AW[fq]AW[bn]AW[bm]AW[dm]AW[eo]AB[do]AW[dn]C[White to play and kill without ko.]AP[goproblems]
(;W[ar]
(;B[br]
(;W[ao];B[ap]
(;W[an];B[dr]
(;W[er];B[ds];W[bs]C[bent four in the cornerRIGHT])
(;W[es]C[NOTTHIS])
(;W[fr]C[NOTTHIS])
(;W[fs]C[NOTTHIS]))
(;W[dr];B[an])
(;W[cn]C[NOTTHIS])
(;W[dp]C[NOTTHIS])
(;W[co]C[NOTTHIS]))
(;W[dr];B[ao])
(;W[ap];B[ao];W[aq];B[dr];W[er];B[ds]))
(;B[ao]C[reference variation];W[br]C[RIGHT])
(;B[aq]C[reference variation];W[ao];B[ap];W[an]C[RIGHT])
(;B[dr]C[reference variation];W[er]
(;B[br];W[ao];B[ap];W[an]C[RIGHT];B[ds];W[bs]C[bent four in the cornerRIGHT])
(;B[aq];W[ds];B[br];W[ao];B[ap];W[an]C[RIGHT])))
(;W[ap];B[ao];W[ar];B[br];W[aq];B[dr];W[er];B[ds])
(;W[ao];B[ar]
(;W[ap];B[bs]LB[aq:A]LB[dr:B])
(;W[bs];B[ap]))
(;W[bs];B[br]
(;W[dr];B[cs])
(;W[ao];B[ap];W[an];B[ar])
(;W[ar];B[ao]))
(;W[br];B[ar]
(;W[bs];B[cs])
(;W[ao];B[bs]))
(;W[dp];B[ar])
(;W[dr];B[br]
(;W[ao];B[ap];W[an];B[ar])
(;W[ar];B[ao])
(;W[cs];B[bs];W[ao];B[ap]
(;W[an];B[ar])
(;W[ar];B[an]))))