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

Lemma ad_list_stutters_app_conv_l :
forall l l':list ad,
ad_list_stutters (l ++ l') = false -> ad_list_stutters l = false.