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

  
    s1.formIntersection(s2)
    
    (s1.contains(.wednesday))
    expectFalse(s1.contains(.tuesday))
    expectFalse(s1.contains(.sunday))
  }

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

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