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

(* l is a list of string. creer_branche takes a list of char and returns a Noeud *)

let creer l =
    let rec make_tree l acc = match l, acc with
    | [], acc' -> acc'
    | word :: l', (a, b) -> make_tree (l' Noeud(a, ref (creer_branche (explode word))::!b))
    in make_tree (l vide)