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

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