type 'a exprat = Vide | Epsilon | Lettre of 'a  | Union of 'a exprat * 'a exprat  | Produit of 'a exprat * 'a exprat | Etoile of 'a exprat ;;(* le langage dˇcrit est-il vide ? *)let rec lambda = function  | Vide -> true  | Epsilon | Lettre _ -> false  | Union(e,e') -> lambda e & lambda e'  | Produit(e,e') -> lambda e || lambda e'  | Etoile _ -> false ;;(* longueur minimale d'un mot du langage dˇcrit *)let rec mu = function  | Vide -> None  | Epsilon ->  Some 0  | Lettre _ -> Some 1  | Union(e,e') -> ( match (mu e,mu e') with    | None,None -> None    | None,Some x' -> Some x'    | Some x,None -> Some x    | Some x,Some x' -> Some(min x x')    ) | Produit(e,e') -> ( match (mu e,mu e') with    | None,_ | _,None -> None    | Some x,Some x' -> Some(x+x')    ) | Etoile _ -> Some 0 ;;(* un mot de longueur minimale du langage dˇcrit *)mu(Union (Lettre `z`,Produit (Lettre `z`,Lettre `z`)));; let rec pc_mot = function  | Vide -> None  | Epsilon ->  Some ""  | Lettre x -> Some(make_string 1 x)  | Union(e,e') -> ( match (pc_mot e,pc_mot e') with    | None,None -> None    | None,Some x' -> Some x'    | Some x,None -> Some x    | Some x,Some x' -> if string_length x < string_length x'       then Some x else Some x'    ) | Produit(e,e') -> ( match (pc_mot e,pc_mot e') with    | None,_ | _,None -> None    | Some x,Some x' -> Some(x^x')    ) | Etoile _ -> Some "" ;;