Hint Resolve apply_heap_lc : lngen.
(** *** Exercise: [apply_stack_lc]  State and prove a lemma called [apply_stack_lc] and add it to the [lngen] hint database so that the proof for [decode_lc] goes through. *)

(* Specifications: *)
(* SOLUTION *)
Lemma apply_stack_lc : forall s h e,
lc_exp e -> lc_exp (apply_stack h s e).