module type ORDERING = sig
    type t
    val (<) : t -> t -> bool
    val (>) : t -> t -> bool
    val (=) : t -> t -> bool
    val compare : t -> t -> int
end

module Int_ordering : ORDERING with type t = int = struct
    type t = int
    let (<) = Pervasives.(<)
    let (>) = Pervasives.(>)
    let (=) = Pervasives.(=)
    let compare = Pervasives.compare
end

let int_ordering = (module Int_ordering : ORDERING with type t = int)

module String_ordering : ORDERING with type t = string = struct
    type t = string
    let (<) x y = String.compare x y < 0
    let (>) x y = String.compare x y > 0
    let (=) x y = String.compare x y = 0
    let compare = String.compare
end

let sort_pair (type a) ordering (x , y) =
    let module Ordering = (val ordering : ORDERING with type t = a) in
    if Ordering.(<) x y then (x, y) else
    if Ordering.(>) x y then (y, x) else (x, y)