prop_emptyIsUnionIdentity :: SetOf e s => s -> Bool
prop_emptyIsUnionIdentity s = (s `union` empty) `sameAs` s
prop_emptyIsUnionIdentity :: SetOf e s => s -> Bool
prop_emptyIsUnionIdentity s = (s `union` empty) `sameAs` s
prop_emptyIsUnionIdentity s = (s `union` empty) `sameAs` s