type comparison = Less | Equal | Greater

module type ORDERED_TYPE =
   sig
     type t
     val compare: t -> t -> comparison
   end
 
module Set =
   functor (Element : ORDERED_TYPE) ->
     struct
       type element = Element.t
       type set = element list

       let empty = []

       let rec adjoin element set =
         match set with
           [] -> [element]
         | head::tail ->
            match Element.compare element head with
              Equal   -> set
            | Less    -> element :: set
            | Greater -> head :: (adjoin element tail)

       let rec is_member element set =
         match set with
           [] -> false
         | head::tail ->
             match Element.compare element head with
               Equal   -> true
             | Less    -> false
             | Greater -> is_member element tail
     end

module OrderedString =
   struct
     type t = string
     let compare x y =
       if x = y
       then Equal
       else if x < y
       then Less
       else Greater
   end

module StringSet = Set(OrderedString)