anchor.diff:          do
               let elements = ResizeArray ()
   
  -            Substring ("The quick brown fox jumps over the lazy dog.", 4, 0)
  +            substring ("The quick brown fox jumps over the lazy dog.", 4, 0)
               |> Substring.iterBack (System.Char.ToUpper >> elements.Add)
   
               elements.ToArray ()
  
positive.code:     let foldBack () : unit =
          // Test case for empty substring.
          do
              (Substring ("The quick brown fox jumps over the lazy dog.", 4, 0), (0L, 0))
              ||> Substring.foldBack (fun c (checksum, index) ->
                  (checksum + int64 index) * int64 c,
                  index + 1)
  
negative.code: 
          // Test case for "normal" usage of this function.
          do
              (substring ("The quick brown fox jumps over the lazy dog.", 4, 15), (0L, 0))
              ||> Substring.foldBack (fun c (checksum, index) ->
                  (checksum + int64 index) * int64 c,
                  index + 1)
  