(** We now state the extensionality property, which expresses that the inclusion relation is antisymmetric (thus being a partial ordering relation between sets).  Equivalently, this property says that two pointed graphs are bisimilar when they have the same elements. *)

Theorem extensionality :
forall (X : Typ1) (A : Rel X) (a : X) (Y : Typ1) (B : Rel Y) (b : Y),
SUB X A a Y B b -> SUB Y B b X A a -> EQV X A a Y B b.