let rec typeof = function
  | Term.Var _ ->
    (* Une variable n'a pas de type *)
    None
  | Term.IntConst _ -> Some Type.Int
  | Term.Binop (t1, _, t2) ->
    (* Les 2 types de l'opération sont égaux *)
    (match typeof t1, typeof t2 with
     | Some (_ as ty1), Some (_ as ty2) -> if ty1 = ty2 then Some ty1 else None
     | _ -> None)
  | Term.Pair (t1, t2) ->
    (* On forme le produit *)
    (match typeof t1, typeof t2 with
     | Some ty1, Some ty2 -> Some (Type.Product (ty1, ty2))
     | _ -> None)
  | Term.Proj (_proj, _t) -> failwith "TODO"
  | Term.Fun (_, _) -> failwith "TODO"
  | Term.App (_t1, _t2) -> failwith "TODO"
;;