anchor.diff:  
       [<Test>]
       let ``[ASet] markingCallback holds gc root``() =
  -        let cset = CSet.ofList [ ]
  -        let mutable x = cset |> ASet.map ((*)2) |> ASet.filter ((>)(-1000)) |> ASet.groupBy id |> AMap.toASet
  -        
  -        let called = ref 0
  -        let reader = x.GetReader()
  -        let mutable y = reader.AddMarkingCallback (fun _ -> called := !called + 1; ) 
  -
  -        y <- null
  -        x <- Unchecked.defaultof<_>
  +        let called, inputSet, reader = GCHelper.``create, register marking and return and make sure that the stack frame dies`` ()
   
           let cnt = 1000
           for i in 0 .. cnt do
  -            transact (fun () -> CSet.add i cset |> ignore)
  +            transact (fun () -> CSet.add i inputSet |> ignore)
               //printfn "should equal i=%d called=%d" i !called
               reader.GetDelta() |> ignore
               should equal  i !called
  
positive.code: 
      module GCHelper =
  
          let createListAndMakeSureTheStackframeIsDead () =
              let cset = CSet.ofList [ ]
              let mutable x = cset |> ASet.map ((*)2) |> ASet.filter ((>)(-1000)) |> ASet.groupBy id |> AMap.toASet
          
  
negative.code:             y <- null
              x <- Unchecked.defaultof<_>
              called, cset
  
          let ``create, register marking and return and make sure that the stack frame dies`` () =
              let cset = CSet.ofList [ ]
              let mutable x = cset |> ASet.map ((*)2) |> ASet.filter ((>)(-1000)) |> ASet.groupBy id |> AMap.toASet
          
              let called = ref 0
              let reader = x.GetReader()
              let mutable y = reader.AddMarkingCallback (fun _ -> called := !called + 1; ) 
  
              y <- null
              x <- Unchecked.defaultof<_>
              called, cset, reader
              
  
      [<Test>]
      let ``[ASet] registerCallback holds gc root``() =
          let called, inputSet = GCHelper.``create, register callback and return and make sure that the stack frame dies`` ()
  
          let cnt = 1000
          for i in 0 .. cnt do
  