module type STACK =
sig
    type element
    type stack
    val empty : stack
    val push : element -> stack -> stack
    val pop : stack -> element*stack
    val to_list : stack -> element list
end

module type IMP =
sig
    type t
end

module ListStack(M : IMP) : STACK with type element = M.t =
struct
    type element = M.t
    type stack = element list
    let empty = []
    let push x s = x :: s
    let pop (x :: xs) = (x, xs)
    let to_list s = s
end
module S = ListStack(
        struct
        type t = int
        end
    )