anchor.diff:      /// and increases the associated reference count.
       /// </summary>
       member x.Invoke (v : 'a) =
  -        if isNull (v :> obj) then
  +        if isNull v then
               match nullCache with
                   | Some (r, ref) -> 
                       ref := !ref + 1
  
positive.code:                 | _ -> failwithf "cannot revoke unknown value: %A" v
  
      member x.RevokeAndGetDeletedTotal (v : 'a) =
          if isNull (v :> obj) then
              match nullCache with
                  | Some (r, ref) -> 
                      ref := !ref - 1
  
negative.code:     /// with the given argument and decreases its reference count.
      /// </summary>
      member x.RevokeAndGetDeleted (v : 'a) =
          if isNull v then
              match nullCache with
                  | Some (r, ref) -> 
                      ref := !ref - 1
  