let withCancellation (ct:CancellationToken) (a:Async<'a>) : Async<'a> = async {
  let! ct2 = Async.CancellationToken
  use cts = CancellationTokenSource.CreateLinkedTokenSource (ct, ct2)
  let tcs = new TaskCompletionSource<'a>()
  use _reg = cts.Token.Register (fun () -> tcs.TrySetCanceled() |> ignore)
  let a = async {
    try
      let! a = a
      tcs.TrySetResult a |> ignore
    with ex ->
      tcs.TrySetException ex |> ignore }
  Async.Start (a, cts.Token)
  return! tcs.Task |> Async.AwaitTask }