sig
  type 'a t
  exception Empty
  val create : unit -> 'a Stack.t
  val push : 'a -> 'a Stack.t -> unit
  val pop : 'a Stack.t -> 'a
  val top : 'a Stack.t -> 'a
  val clear : 'a Stack.t -> unit
  val length : 'a Stack.t -> int
  val iter : f:('a -> unit) -> 'a Stack.t -> unit
end