anchor.diff:  *)
   
   //
  -exception UndefinedException
  -
  -//
   [<NoEquality; NoComparison>]
   type internal LazyCellStatus<'T> =
       //
  
positive.code:             lock this <| fun () ->
                  match status with
                  | Delayed f ->
                      status <- Exception UndefinedException
                      try
                          let res = f ()
                          status <- Value res
  
negative.code:     LazyList<'T> internal (initialStatus) =
      //
      static let emptyList = LazyList (Value Empty)
      
      /// The status for undefined values.
      static let undefinedValue =
          System.InvalidOperationException "The value of the LazyList cell is undefined."
          :> exn
          |> Exception
  
      //
      let mutable status : LazyCellStatus<'T> = initialStatus
  