   
     s1.formSymmetricDifference(s2)
 
-    (s1.contains(.tuesday))
+    expectTrue(s1.contains(.tuesday))
     expectFalse(s1.contains(.wednesday))
     expectFalse(s1.contains(.sunday))
   }

    let s2: Days = [.tuesday, .wednesday, .friday]
  
    s1.formUnion(s2)
    (s1.contains(.monday))
    (s1.contains(.tuesday))
    expectFalse(s1.contains(.sunday))
  }
}

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