anchor.diff:  
           list.LongLength
   
  +    /// <summary>
       /// Returns the evaluated ("forced") length of the list -- i.e., the number
  -    /// of elements which have already been evaluated. Unlike LazyList.length,
  +    /// of elements which have already been evaluated. Unlike <c>LazyList.length</c>,
       /// this does not force evaluation of any cells and always terminates.
  +    /// </summary>
       [<CompiledName("ForcedLength")>]
       let forcedLength (list : LazyList<'T>) =
           // Preconditions
  
positive.code:         checkNonNull "list1" list1
          checkNonNull "list2" list2
  
          lzy <| fun () ->
              match list1.Value, list2.Value with
              | Cons (hd1, tl1), Cons (hd2, tl2) ->
                  consCell (mapping hd1 hd2) (map2 mapping tl1 tl2)
  
negative.code:         if count < 0 then
              argOutOfRange "count" "Cannot skip a negative number of elements."
  
          LazyList<_>.CreateLazy <| fun () ->
              skipCell count list
  
      /// Apply the given function to each element of the collection.
  