intuition.
simpl.
eapply filter_In.
split.
eapply in_getUnique.
eapply in_flatten.
econstructor.
split.
eapply in_map_iff.
econstructor.
split.
eapply eq_refl.
eapply in_getAllBvectors.
simpl.
left.
eapply bvToNat_natToBv_inverse.
eapply lognat_monotonic.
trivial.
unfold ltNatBool.
destruct (lt_dec x k); intuition.
Qed.

(* Specifications: *)
Lemma RndNat_support_length :
forall n,
length (getSupport (RndNat n)) = n.