OCaml中嵌套签名的示例？

module type Stack = sig
  type 'a t
  val empty : 'a t
  val push : 'a -> 'a t -> 'a t
  val pop : 'a t -> 'a t
  val top : 'a t -> 'a option
end

module type Queue = sig
  type 'a t
  val empty : 'a t
  val enqueue : 'a -> 'a t -> 'a t
  val dequeue : 'a t -> 'a t
  val front : 'a t -> 'a option
end

module type StackQueue = sig
  module S : Stack
  module Q : Queue

  val enqueue : 'a -> 'a Q.t -> 'a Q.t
  val dequeue : 'a Q.t -> 'a Q.t
  val front : 'a Q.t -> 'a option
end

module StackQueueImpl : StackQueue = struct
  module S = struct
    type 'a t = 'a list
    let empty = []
    let push x s = x :: s
    let pop = function
      | [] -> []
      | _ :: s -> s
    let top = function
      | [] -> None
      | x :: _ -> Some x
  end

  module Q = struct
    type 'a t = 'a list
    let empty = []
    let enqueue x q = q @ [x]
    let dequeue = function
      | [] -> []
      | _ :: q -> q
    let front = function
      | [] -> None
      | x :: _ -> Some x
  end

  let enqueue x q = Q.enqueue x q
  let dequeue q = Q.dequeue q
  let front q = Q.front q
end

let q = StackQueueImpl.Q.empty
let q = StackQueueImpl.enqueue 1 q
let q = StackQueueImpl.enqueue 2 q
let q = StackQueueImpl.enqueue 3 q
let q = StackQueueImpl.dequeue q
let front = StackQueueImpl.front q