 OptionSetTests.test("IsEmpty.EmptySet") {
   var s: Days = []
   
-  (s.isEmpty)
+  expectTrue(s.isEmpty)
   
   s.insert(.monday)
   

  
  let member1 = s.update(with: .tuesday)

  (s.contains(.monday))
  expectTrue(s.contains(.tuesday))
  expectFalse(s.contains(.sunday))
  expectNil(member1)

  let s2: Days = [.monday, .tuesday, .wednesday]

  s1.formSymmetricDifference(s2)
  expectTrue(s1.contains(.wednesday))
  expectFalse(s1.contains(.thursday))
}

