Module BAF := BA_Facts BA.(* Now, we prove the axioms about relativization.
     Again, this mostly involves packing and unpacking
     the canonTree type and pushing around mkCanon.
   *)

    Definition rel (a x:t) : t.

Lemma relativ_tree_empty : forall a x, nonEmptyTree a -> nonEmptyTree x -> nonEmptyTree (relativ_tree a x).