(;AB[dr]AB[er]AB[eq]AB[ep]AB[fp]AB[gq]AB[gr]AB[hr]AW[ir]AW[hq]AW[hp]AW[gp]AW[jq]AW[fo]AW[eo]AW[dp]AW[dq]AW[do]AW[cr]AW[ds]AB[cq]AW[bq]AB[br]LB[cq:1]LB[bq:2]LB[br:3]C[After black 1, w 2 then b 3, it is a mistake. Show him why!]AP[goproblems] (;W[cp];B[bs] (;W[hs];B[cs];W[ar];B[cq]C[Yes, it is a ko. Black could live unconditionally (see the problem It's Alive!)RIGHT]) (;W[ar];B[hs];W[fs];B[fr];W[es];B[gs]C[Black lives]) (;W[cs] (;B[es];W[hs];B[cq];W[cr];B[cs];W[ar];B[cq]C[Correct, a ko :)RIGHT]) (;B[hs];W[fs];B[es];W[fr];B[cq];W[cr];B[cs];W[ar];B[cq]C[Correct, ko :)RIGHT])) (;W[es];B[cs]C[];W[ar];B[fs];W[ds];B[hs]) (;W[fs];B[fr];W[es];B[gs])) (;W[cs];B[es] (;W[cp];B[hs]) (;W[hs];B[bs] (;W[cr];B[cp]) (;W[cp];B[cr]))))