I missed it in one of my game in kgs, and loose the game. But it is a simple move.
(;AP[goproblems]AB[ap]AB[bq]AB[cq]AW[aq]AW[ar]AW[br]AW[cr]AW[dq]AW[eq]AW[fq]AW[fp]AB[dp]AB[ep]AB[eo]AB[bo]AB[co]AB[dn]AB[fn]AB[fo]AB[gn]AB[hn]AW[hm]AW[gm]AW[fm]AW[em]AW[dm]AW[cm]AW[bm]AW[an]AW[in]AW[io]AW[ip]AW[il]AW[hq]C[white to kill](;W[bp];B[cp](;W[ao]C[b4 is the keyRIGHT])(;W[bn];B[ao](;W[cn];B[en])(;W[en];B[cn]))(;W[en];B[cn](;W[ao]C[RIGHT])(;W[bn];B[ao])(;W[ho];B[ao])(;W[go];B[ao]))(;W[cn];B[en](;W[ao]C[RIGHT])(;W[bn];B[ao])(;W[go];B[ao])(;W[ho];B[ao])))(;W[ao];B[bp])(;W[cn];B[en](;W[bp];B[cp](;W[ao]C[b4 is the key pointRIGHT])(;W[bn];B[ao])(;W[go];B[ao])(;W[ho];B[ao]))(;W[ao];B[bp])(;W[cp];B[bp])(;W[ho];B[ao])(;W[go];B[ao])(;W[bn];B[ao])(;W[gp];B[ao]))(;W[en];B[cn](;W[bp];B[cp];W[ao]C[b4 is the key pointRIGHT])(;W[cp];B[bp])(;W[ao];B[bp])(;W[bn];B[ao])(;W[go];B[ao])(;W[ho];B[ao]))(;W[cp];B[bp])(;W[gp];B[ao])(;W[bn];B[ao](;W[en];B[cn])(;W[cn];B[en])(;W[cp];B[bp](;W[cn];B[en])(;W[en];B[cn]))(;W[bp];B[cp](;W[cn];B[en])(;W[en];B[cn])))(;W[ho];B[ao](;W[cn];B[en](;W[cp];B[bp])(;W[bp];B[cp]))(;W[en];B[cn](;W[cp];B[bp])(;W[bp];B[cp]))(;W[bp];B[cp](;W[cn];B[en])(;W[en];B[cn]))(;W[cp];B[bp](;W[cn];B[en])(;W[en];B[cn])))(;W[go];B[ao](;W[cn];B[en](;W[cp];B[bp])(;W[bp];B[cp]))(;W[en];B[cn](;W[cp];B[bp])(;W[bp];B[cp]))(;W[cp];B[bp](;W[cn];B[en])(;W[en];B[cn]))(;W[bp];B[cp](;W[cn];B[en])(;W[en];B[cn]))))