intros (nx, ex) n.
unfold Dcompare in |- *; simpl in |- *.
symmetry  in |- *.
replace (two_p (n - Zmin ex n) * 0)%Z with (two_p (ex - Zmin ex n) * 0)%Z.
apply Zmult_compare_compat_l.
apply two_p_gt_ZERO.
generalize (Zle_min_l ex n); generalize (Zmin ex n); intro; omega.
do 2 rewrite Zmult_0_r; reflexivity.
Qed.

(* Specifications: *)
Lemma Deq_shift :
forall (x y : diadic) (n : Z), Deq x y -> Deq (Dshift n x) (Dshift n y).