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