type lambda_exp = Var of string
  | Lambda of string * lambda_exp
  | Appl of lambda_exp * lambda_exp;;

let longer s1 s2 =
  if ((String.length s1) > (String.length s2)) then s1 else s2;;

let rec longest exp =
  match exp with
    Var s -> s
  | Lambda(s,exp1) -> longer s (longest exp1)
  | Appl(exp1,exp2) -> longer (longest exp1) (longest exp2);;

let fresh exp = (longest exp)^"f";;