(** Emitter that stores signals into a list, in reverse order (most recent
    signals first). *)
let to_list (l : 'a list ref) : 'a Emitter.t =
  let closed = Atomic.make false in
  {
    emit = (fun sigs -> l := List.rev_append sigs !l);
    tick = (fun ~now:_ -> ());
    closed = (fun () -> Atomic.get closed);
    flush_and_close = (fun () -> Atomic.set closed true);
  }