anchor.diff:  *)
   
   //
  -exception UndefinedException
  -
  -//
   [<NoEquality; NoComparison>]
   type internal LazyCellStatus<'T> =
       //
  
positive.code:     LazyList<'T> internal (initialStatus) =
      //
      static let emptyList = LazyList (Value Empty)
  
      //
      let mutable status : LazyCellStatus<'T> = initialStatus
  
negative.code:             lock this <| fun () ->
                  match status with
                  | Delayed f ->
                      status <- undefinedValue
                      try
                          let res = f ()
                          status <- Value res
  