anchor.diff:          let toSeq s =
               s |> Seq.unfold (fun list ->
                   match getCell list with
  -                | CellEmpty ->
  +                | Empty ->
                       None
  -                | CellCons (hd, tl) ->
  +                | Cons (hd, tl) ->
                       Some (hd, tl))
           (toSeq this).GetEnumerator ()
               
  
positive.code: // so CellEmpty would be represented as null?
  and [<NoEquality; NoComparison>]
      internal LazyListCell<'T> =
          | CellEmpty
          | CellCons of 'T * LazyList<'T>
  
  /// LazyLists are possibly-infinite, cached sequences.  See also IEnumerable/Seq for
  /// uncached sequences. LazyLists normally involve delayed computations without 
  
negative.code:     [<CompiledName("Tail")>]
      let tail (list : LazyList<'T>) =
          match getCell list with
          | Cons (_, tl) -> tl
          | Empty ->
              invalidArg "s" "The list is empty."
  
      /// Test if a list is empty.
  