In the game, black missed an opportunity here. Can you see it?
(;GM[1]FF[4]CA[UTF-8]AP[CGoban:2]ST[2]
RU[Japanese]SZ[19]KM[0.00]TM[]
PW[Weiß]PB[Schwarz]AW[mb][ob][mc][nc][pc][qc][qd][re]AB[kb][nb][lc][oc][ld][md][od][pd][qe][qf][rf]C[Black to play]PL[B]
(;B[pb]CR[pb]
;W[na]CR[na]
;B[qb]CR[qb]
;W[rb]CR[rb]
;B[rc]CR[rc]
;W[rd]CR[rd]
(;B[sb]CR[sb]
(;W[sc]CR[sc]
;B[ra]CR[ra]C[This is a two step ko RIGHT CHOICE])
(;W[ra]CR[ra]
;B[sd]LB[sc:A]CR[sd]C[ This is a direct ko. White should have played S19 at A! RIGHT CHOICE]))
(;B[ra]CR[ra]
;W[sb]LB[sc:A]CR[sb]C[Of course white doesn't allow a ko by playing T18 at A !]))
(;B[rd]CR[rd]
;W[rc]CR[rc]C[This is not small, but black can do better])
(;B[rc]CR[rc]C[This only works if white makes a mistake ]
(;W[rd]CR[rd]C[This is a mistake, black can reach the solution now. White shouldn't connect!]
;B[pb]CR[pb]
;W[na]CR[na]
;B[qb]CR[qb]
;W[rb]CR[rb]
;B[sb]CR[sb]
(;W[sc]CR[sc]
;B[ra]CR[ra]C[After the white mistake earlier, the same position as in the solution is reached, a two step ko for black. CHOICE])
(;W[ra]CR[ra]
;B[sd]CR[sd]C[After the white mistake earlier, the same end position as in the solution is reached. W S19 was another mistake: W T17 is better! Now black has a direct ko. CHOICE]))
(;W[rb]CR[rb]C[This W2 ist better than connecting]
;B[rd]CR[rd]
;W[pb]CR[pb]C[ White lost some points in the corner, but there is no danger of losing the whole group now. CHOICE])))