Fixpoint Elems (l:list ad) : FSet :=
    match l with
    | nil => M0 unit
    | a :: l' => MapPut _ (Elems l') a tt
    end.

Lemma ad_list_Elems : forall l l':list ad, MapCard _ (Elems l) = MapCard _ (Elems l') -> length l = length l' -> ad_list_stutters l = ad_list_stutters l'.