function App() {
  return (
    <ScrollStage pages={2}>
      <Actor start={0} end={0.5}>
        <p>I'll be here from start to 50%</p>
      </Actor>
      <Actor start={0.5}>
        <p>I'll be here from 50% on.</p>
      </Actor>
    </ScrollStage>
  )
}
  
// hooks access info about the progress
// for an actor (relative to stage) or the
// the entire stage
let actor = useActor()
let stage = useStage()