intuition.
unfold RndNat in *.
simpl in *.
eapply filter_In in H.
intuition.
unfold ltNatBool in *.
destruct (lt_dec x n); congruence.
Qed.
Hint Resolve well_formed_RndNat : wftac.

Lemma RndNat_unchecked_lt_support : forall n v,
v < n ->
In v (getSupport (RndNat_unchecked n)).