type expr = 
  [ `Abs of string * expr
  | `App of expr * expr
  ]

class type exp = 
object
  method eval : (string, exp) Hashtbl.t -> expr
end;;

class app e1 e2 : exp = 
object
  val l = e1 
  val r = e2
  method eval env = 
      match l with
    | `Abs(var,body) ->
        Hashtbl.add env var r;
        body
    | _ -> `App(l,r);
end
