intros ni1 ni2 ni3 eqv12.
generalize ni3; clear ni3.
elim eqv12; clear eqv12 ni1 ni2.
intros; assumption.
intros x ni1 ni2 eqv12 ih ni3 eqv23.
inversion_clear eqv23.
apply Eqv_NI_Cons_NN.
apply ih; assumption.
apply Eqv_NI_Cons_ND.
apply ih; assumption.
intros x k ni1 ni2 eqv12 ih ni3 eqv23.
inversion_clear eqv23.
apply Eqv_NI_Cons_DN.
apply ih; assumption.
apply Eqv_NI_Cons_DD.
apply ih; assumption.
intros x k k' ni1 ni2 eqv12 ih ni3 eqv23.
inversion_clear eqv23.
apply Eqv_NI_Cons_DN.
apply ih; assumption.
apply Eqv_NI_Cons_DD.
apply ih; assumption.
intros x k ni1 ni2 eqv12 ih ni3 eqv23.
inversion_clear eqv23.
apply Eqv_NI_Cons_NN.
apply ih; assumption.
apply Eqv_NI_Cons_ND.
apply ih; assumption.
Qed.

(* Specifications: *)
Lemma le_eqv : forall ni1 ni2 : nested_imps, le_ni ni1 ni2 -> eqv_ni ni1 ni2.
intros ni1 ni2 le12; elim le12; clear le12 ni1 ni2.