module type Format = sig
    type t
    val to_int : t -> int
end

module type Maker = functor (F : Format) -> sig
    type t
    val of_string : string -> t
    val of_format : t -> string
    val length : t -> int
end

module FirstFormat =
    struct
        type t = string
        let to_int fmt = 2
    end

module SecondFormat =
    struct
        type t = string
        let to_int fmt = 1
    end

module Make (F : Format) : Maker = struct
    type t = F.t
    let of_string str = str
    let of_format fmt = fmt
    let length fmt = String.length (of_format fmt)
end