intros ni1 ni2 le12.
elim le12; clear le12 ni1 ni2.
trivial.
intros x ni1 ni2 le ih; simpl in |- *.
apply le_n_S; assumption.
intros x k ni1 ni2 le ih; simpl in |- *.
apply le_trans with (count_undecs ni2).
assumption.
apply le_n_Sn.
intros x k ni1 ni2 le ih; simpl in |- *.
assumption.
Qed.

(* Specifications: *)
Lemma count_undecs_rev_app :
forall (dni : decorated_nested_imps) (ni : nested_imps),
count_undecs (rev_app dni ni) = count_undecs ni.