Function scene_answer 

pub fn scene_answer(
    op: Op,
    session: &dyn Session,
    ask: SceneAsk<'_>,
    record: Option<SessionPlayed>,
    differs: bool,
) -> Answer<SceneOutput>

The answer op gives with session’s scene, asked ask, and how far its record played, record: it passes if there is a scene and the record played no differently (differs).