 S’s being able to do otherwise is distinct from its merely being possible that S do otherwise. Once Lisa the skydiver jumps out her plane, it is (logically) possible that she float in midair rather than fall. But it’s not in her power to do so: the world in which Lisa floats is not accessible to her. And if Kurt is brainwashed to steal John’s lunch, then while it’s possible that he refrain, he nevertheless can’t. Something must be added to S’s possibly doing something else to ensure it’s within S’s power to do so.