proposition: Lemma perm_nb_occ: forall (l l' : list A), perm l l' -> forall (a : A), nb_occ a l = nb_occ a l'.
proof: Proof. intros l l' H; elim H; auto. intros l'0; intros; transitivity (nb_occ a l'0);auto. apply transpose_nb_occ; auto. Qed.
imports: List Arith