anchor.diff:      [<TestCase>]
       let get () : unit =
           do
  -            let substr = Substring ("Hello World!", 3, 5)
  +            let substr = substring ("Hello World!", 3, 5)
               
               Substring.get substr 1
               |> should equal 'o'
  
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 ()
  