type exp = Var of int
      | Lit of int
      | Lam of (int * exp)
      | App of (exp * exp)
      | Plus of (exp * exp)
      | Minus of (exp * exp)
      | Ifz of (exp * exp * exp)

and result = Resint of int
          | Closure of (int * exp * env)
and env = Empty
       | Extended of (int * result * env)
;;

(* >>>>>> This definition must remain intact <<<<<<< *)
let sum10 = App(Lam(19, App(App(Var 19, Var 19),
                            Lit 10)),
                Lam(18,
                    Lam(17, Ifz(Var 17,
                                Lit 0,
                                Plus(Var 17,
                                     App(App(Var 18, Var 18),
                                         Minus(Var 17, Lit 1)))))))

;;

exception Bad
;;

(* >>>>>> Most of your work is to implement eval <<<<<<< *)
let rec eval(x, e) = raise Bad;

eval (sum10, Empty)
;;