Fixpoint ad_in_list (a:ad) (l:list ad) {struct l} : bool :=
    match l with
    | nil => false
    | a' :: l' => orb (Neqb a a') (ad_in_list a l')
    end.Fixpoint ad_list_stutters (l:list ad) : bool :=
    match l with
    | nil => false
    | a :: l' => orb (ad_in_list a l') (ad_list_stutters l')
    end.

Lemma ad_list_stutters_has_circuit : forall l:list ad, ad_list_stutters l = true -> {x : ad & {l0 : list ad & {l1 : list ad & {l2 : list ad | l = l0 ++ x :: l1 ++ x :: l2}}}}.