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