anchor.diff:              testPropertyWithConfig config10k "prop insert and tryFind" <|
                   fun k t -> IntMap.tryFind k (IntMap.insert k () t) <> None
   
  -            ptestPropertyWithConfig config10k "prop insert and delete" <|
  +            testPropertyWithConfig config10k "prop insert and delete" <|
                   fun k t -> IntMap.tryFind k t = None ==> (IntMap.delete k (IntMap.insert k () t) = t)
   
  -            ptestPropertyWithConfig config10k "prop delete non member" <|
  +            testPropertyWithConfig config10k "prop delete non member" <|
                   fun k t -> IntMap.tryFind k t = None ==> (IntMap.delete k t = t)
   
               testPropertyWithConfig config10k "prop append" <|
  
positive.code:                 fun (xs: int list) ->
                      List.sort (List.ofSeq (Seq.distinct xs)) = [ for (x,()) in IntMap.toList (IntMap.ofList [ for x in xs do yield (x,()) ]) do yield x ]
  
              ptestPropertyWithConfig config10k "prop alter" <|
                  fun t k ->
                      let f = function | Some () -> None | None -> Some ()
                      let t' = IntMap.alter f k t
  
negative.code:     [<Tests>]
      let testIntMapProperties =
  
          let except (xs: _ seq) ys = xs.Except(ys)
          let intersect (xs: _ seq) ys = xs.Intersect(ys)
          let mapOption (f: 'a -> 'b option) l = List.foldBack (fun x xs -> match f x with Some v -> v::xs | None -> xs) l []
  