Fixpoint undef (ll: list loc) (m: t) {struct ll} : t :=
    match ll with
    | nil => m
    | l1 :: ll' => undef ll' (set l1 Vundef m)
    end.

(* Specifications: *)
Lemma guo: forall ll l m, Loc.notin l ll -> (undef ll m) l = m l.
Lemma gus: forall ll l m, In l ll -> (undef ll m) l = Vundef.