comiluv: B S2 is the only necessary move because (no ko) is not stated in the problem
caranthir: "No ko" means that the killing must be unconditional and not depend on winning a ko using some imaginary non-local ko threat(s). Therefore if S2, T2, the attacker must then continue with S1 in order to prevent ko.
"No ko" means that the killing must be unconditional and not depend on winning a ko using some imaginary non-local ko threat(s). Therefore if S2, T2, the attacker must then continue with S1 in order to prevent ko.