Program Definition union (s1 s2: t) : t := R.union s1 s2.

(* Specifications: *)
Theorem In_union: forall x s1 s2, In x (union s1 s2) <-> In x s1 \/ In x s2.
Theorem In_union: forall x s1 s2, In x (union s1 s2) <-> In x s1 \/ In x s2.