type arbre =  Noeud of bool ref  * (char * arbre) list ref

                                      *)
let vide () = Noeud(ref false,ref[])


let creer l =
    let rec make_tree strs acc = match strs, acc with
    | [], acc' -> acc'
    | word::rest, acc' -> make_tree rest acc'
  in make_tree l vide()

let creer l =
    let rec make_tree strs acc = match strs, acc with
    | [], acc' -> acc'
    | word::rest, Noeud(a, b) as acc' -> make_tree rest acc'
  in make_tree l vide()