let a = Animation(of: { view.frame.origin.x += 10 }, duration: 1)
let b = a.with(animations: { view.frame.origin.x -= 10 })
let timeline = AnimationTimeline(a, b, b, a).start() { (finished) in
  guard finished else {
    // ...
    return
  }
  print("done")
}
// ...
timeline.needsToCancel = true

var isCancelled = false
let handleCancelled = {
  // ...
}
let a = { view.frame.origin.x += 10 }
let b = { view.frame.origin.x -= 10 }
let start = {
  UIView.animate(withDuration: 1, animations: a) { _ in
    if isCancelled {
      handleCancelled()
      return
    }
    UIView.animate(withDuration: 1, animations: b) { _ in
      if isCancelled {
        handleCancelled()
        return
      }
      UIView.animate(withDuration: 1, animations: b) { _ in
        if isCancelled {
          handleCancelled()
          return
        }
        UIView.animate(withDuration: 1, animations: a) { _ in
          print("done")
        }
      }
    }
  }
}
start()
// ...
isCancelled = true

var z = b
z.animations = { /* ... */ }
z.type = .keyframed(options: [])
// ...
timeline.append(z)