anchor.diff:      let actual = fromList [1; 2; 3] |> insert 3
       let expected = fromList [1; 2; 3]
       Assert.That(actual, Is.EqualTo(expected))
  -    
  -[<Test>]
  -[<Ignore("Remove to run test")>]
  -let ``Empty set is a subset of another empty set`` () =
  -    let left = empty
  -    let right = empty
  -    Assert.That(isSubsetOf left right, Is.True)
  -    
  -[<Test>]
  -[<Ignore("Remove to run test")>]
  -let ``Empty set is a subset of non-empty set`` () =
  -    let left = empty
  -    let right = singleton 1
  -    Assert.That(isSubsetOf left right, Is.True)
  -    
  -[<Test>]
  -[<Ignore("Remove to run test")>]
  -let ``Non-empty set is not a subset of empty set`` () =
  -    let left = singleton 1
  -    let right = empty
  -    Assert.That(isSubsetOf left right, Is.False)
  -    
  -[<Test>]
  -[<Ignore("Remove to run test")>]
  -let ``Set is a subset of set with exact same elements`` () =
  -    let left = fromList [1; 2; 3]
  -    let right = fromList [1; 2; 3]
  -    Assert.That(isSubsetOf left right, Is.True)
  -    
  -[<Test>]
  -[<Ignore("Remove to run test")>]
  -let ``Set is a subset of larger set with same elements`` () =
  -    let left = fromList [1; 2; 3]
  -    let right = fromList [4; 1; 2; 3]
  -    Assert.That(isSubsetOf left right, Is.True)
  -    
  -[<Test>]
  -[<Ignore("Remove to run test")>]
  -let ``Set is not a subset of set that does not contain its elements`` () =
  -    let left = fromList [1; 2; 3]
  -    let right = fromList [4; 1; 3]
  -    Assert.That(isSubsetOf left right, Is.False)
  -        
  -[<Test>]
  -[<Ignore("Remove to run test")>]
  -let ``The empty set is disjoint with itself`` () =
  -    let left = empty
  -    let right = empty
  -    Assert.That(isDisjointFrom left right, Is.True)    
  -    
  -[<Test>]
  -[<Ignore("Remove to run test")>]
  -let ``Empty set is disjoint with non-empty set`` () =
  -    let left = empty
  -    let right = singleton 1
  -    Assert.That(isDisjointFrom left right, Is.True)
  -    
  -[<Test>]
  -[<Ignore("Remove to run test")>]
  -let ``Non-empty set is disjoint with empty set`` () =
  -    let left = singleton 1
  -    let right = empty
  -    Assert.That(isDisjointFrom left right, Is.True)
  -    
  -[<Test>]
  -[<Ignore("Remove to run test")>]
  -let ``Sets are not disjoint if they share an element`` () =
  -    let left = fromList [1; 2]
  -    let right = fromList [2; 3]    
  -    Assert.That(isDisjointFrom left right, Is.False)
  -    
  -[<Test>]
  -[<Ignore("Remove to run test")>]
  -let ``Sets are disjoint if they share no elements`` () =
  -    let left = fromList [1; 2]
  -    let right = fromList [3; 4]
  -    Assert.That(isDisjointFrom left right, Is.True)
               
   [<Test>]
   [<Ignore("Remove to run test")>]
  
positive.code: let ``Detect if the element is not in the set`` () =
      let actual = fromList [1; 2; 3]
      Assert.That(contains 4 actual, Is.False)
  
  [<Test>]
  [<Ignore("Remove to run test")>]
  
negative.code:     let left = fromList [1; 3]
      let right = fromList [2; 3]
      let expected = fromList [3; 2; 1]
      Assert.That(union left right = expected)