anchor.diff:  
   [<TestCase>]
   let containsKey () : unit =
  +    [(5, 'a'); (3, 'b')]
  +    |> IntMap.ofList
  +    |> IntMap.containsKey 5
  +    |> should be True
  +
       [| (5, 'a'); (3, 'b'); (11, 'f'); (2, 'd');
           (17, 'a'); (4, 'g'); (12, 'b'); (14, 'c'); |]
       |> IntMap.ofArray
  
positive.code: 
  [<TestCase>]
  let union () : unit =
  //    let a = IntMap.ofList [(5, "a"); (3, "b")]
  //    let b = IntMap.ofList [(5, "A"); (7, "C")]
  //    let c = IntMap.union a b
  //    let d = IntMap.ofList [(3, "b"); (5, "a"); (7, "C")]
  
      IntMap.union
          (IntMap.ofList [(5, "a"); (3, "b")])
          (IntMap.ofList [(5, "A"); (7, "C")])
      |> should equal (
          IntMap.ofList [(3, "b"); (5, "a"); (7, "C")])
  
  [<TestCase>]
  let ofSeq () : unit =
  
negative.code: 
  [<TestCase>]
  let map () : unit =
      [| (5, 'a'); (3, 'b'); (11, 'f'); (2, 'd');
          (17, 'a'); (4, 'g'); (12, 'b'); (14, 'c'); |]
      |> IntMap.ofArray
      |> IntMap.map (sprintf "%i:%c")
      |> should equal (IntMap.ofArray
          [| (5, "5:a"); (3, "3:b"); (11, "11:f"); (2, "2:d");
              (17, "17:a"); (4, "4:g"); (12, "12:b"); (14, "14:c"); |])
  
  //[<TestCase>]
  //let filter () : unit =
  