Proof using.
intros.
induction l; simpl in *; auto.
invc_NoDup.
concludes.
intro.
break_or_hyp; auto with datatypes.
Qed.

(* Specifications: *)
Lemma count_occ_app :
forall l l' (a : A),
count_occ A_eq_dec (l ++ l') a = count_occ A_eq_dec l a + count_occ A_eq_dec l' a.