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 map () : unit =
      [(5, "a"); (3, "b")]
      |> IntMap.ofList
      |> IntMap.map (sprintf "%i:%s")
      |> should equal (
          IntMap.ofList [(5, "5:a"); (3, "3:b")])
  
  //[<TestCase>]
  //let filter () : unit =
  
negative.code: 
  [<TestCase>]
  let union () : unit =
      let x = IntMap.ofArray [| (4, 'g'); (5, 'a'); (11, 'f'); (14, 'c'); (17, 'a'); |]
      let y = IntMap.ofArray [| (2, 'd'); (3, 'b'); (4, 'G'); (11, 'F'); (12, 'b'); |]
      let z = IntMap.ofArray [| (2, 'd'); (3, 'b'); (4, 'G'); (5, 'a'); (11, 'F'); (12, 'b'); (14, 'c'); (17, 'a'); |]
      let result_xy = IntMap.union x y
      let result_yx = IntMap.union y x
  
      let qq =
          x
          |> IntMap.add 2 'd'
          |> IntMap.add 3 'b'
          |> IntMap.add 4 'G'
          |> IntMap.add 11 'F'
          |> IntMap.add 12 'b'
  
  
  //    let x = IntMap.ofArray [| (5, 'a'); (11, 'f'); (17, 'a'); (4, 'g'); (14, 'c'); |]
  //    let y = IntMap.ofArray [| (3, 'b'); (11, 'F'); (2, 'd'); (4, 'G'); (12, 'b'); |]
  //    let z = IntMap.union y x
  //    let w = IntMap.ofArray [| (5, 'a'); (3, 'b'); (11, 'F'); (2, 'd'); (17, 'a'); (4, 'G'); (12, 'b'); (14, 'c'); |]
  
      IntMap.union
          (IntMap.ofArray [| (3, 'b'); (11, 'F'); (2, 'd'); (4, 'G'); (12, 'b'); |])
          (IntMap.ofArray [| (5, 'a'); (11, 'f'); (17, 'a'); (4, 'g'); (14, 'c'); |])
      |> should equal (
          IntMap.ofArray [| (5, 'a'); (3, 'b'); (11, 'F'); (2, 'd'); (17, 'a'); (4, 'G'); (12, 'b'); (14, 'c'); |])
  
  [<TestCase>]
  let ofSeq () : unit =
  