anchor.diff:  
   [<Test>]
   let ``prop append and singleton``() =
  +    registerGen.Force()
       fsCheck (fun t k x -> IntMap.append (IntMap.singleton k x) t = IntMap.insert k x t)
   
   [<Test>]
  
positive.code: 
  [<Test>]
  let ``prop insert and tryFind``() =
      fsCheck (fun k t -> IntMap.tryFind k (IntMap.insert k () t) <> None)
  
  [<Test>]
  
negative.code: 
  [<Test>]
  let ``prop isEmpty``() =
      registerGen.Force()
      fsCheck (fun m -> IntMap.isEmpty m = (IntMap.size m = 0))
  
  [<Test>]
  