Definition UNION (X : Typ1) (A : Rel X) (a : X) (z' z : opt X) :=
  forall E : Prop,
  (forall x x' : X,
   eq (opt X) z' (some X x') -> eq (opt X) z (some X x) -> A x' x -> E) ->
  (forall x x' : X,
   eq (opt X) z' (some X x') -> eq (opt X) z (none X) -> A x' x -> A x a -> E) ->
  E.

Lemma UNION_eqv : forall (X : Typ1) (A : Rel X) (a x : X), EQV X A x (opt X) (UNION X A a) (some X x).