effect Get_next : (unit -> unit) option
effect Add_to_queue : (unit -> unit) -> unit

let queue initial = handler
  | effect Get_next k ->
    ( fun queue -> match queue with
        | [] -> (continue k None) []
        | hd::tl -> (continue k (Some hd)) tl )
  | effect (Add_to_queue y) k -> ( fun queue -> (continue k ()) (queue @ [y]))
  | x -> ( fun _ -> x)
  | finally x -> x initial
;;