Function UNIVERSE

The universe of a relation is the set of all objects occurring in some list contained in that relation.
Arity: 2
Axioms:
(=> (Universe ?R ?Set)
    (And (Relation ?R)
         (= ?Set
            (Setofall ?X
                      (Exists (?List)
                              (And (Member ?List ?R) (Item ?X ?List)))))))