Hint Resolve find_letvar_valid: rtlg.
(** Properties of [add_var]. *)

Lemma add_vars_letenv:
forall il map1 s1 rl map2 s2 i,
add_vars map1 il s1 = OK (rl, map2) s2 i ->
map2.(map_letvars) = map1.(map_letvars).