Hint Resolve map_valid_incr: rtlg.
(** * Properties of basic operations over the state *)
(** Properties of [add_instr]. *)

(* Specifications: *)
Lemma add_instr_at:
forall s1 s2 incr i n,
add_instr i s1 = OK n s2 incr -> s2.(st_code)!n = Some i.