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

Lemma ad_list_stutters_permute : forall l l':list ad, ad_list_stutters (l ++ l') = ad_list_stutters (l' ++ l).