module type QUEUE = 
sig
  type 'a t
  exception Empty
  val create : unit -> 'a t
  val add : 'a -> 'a t -> unit
  val push : 'a -> 'a t -> unit
  val take : 'a t -> 'a
  val pop : 'a t -> 'a
  val peek : 'a t -> 'a
  val top : 'a t -> 'a
  val clear : 'a t -> unit
  val copy : 'a t -> 'a t
  val is_empty : 'a t -> bool
  val length : 'a t -> int
  val iter : ('a -> unit) -> 'a t -> unit
  val fold : ('a -> 'b -> 'a) -> 'a -> 'b t -> 'a
  val transfer : 'a t -> 'a t -> unit
end


module Queue = struct
  include Queue

  let string_of str q =
    let sl = fold 
      (fun s e -> (str e) :: s)
      []
      q
    in
      String_of.string_of_list (fun s -> s) sl
      
end