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_nonid : forall a x1 x2, nonEmptyTree a -> a = relativ_tree a (Node x1 x2) -> False.