anchor.diff:      let foldBack () : unit =
           // Test case for empty substring.
           do
  -            (Substring ("The quick brown fox jumps over the lazy dog.", 4, 0), (0L, 0))
  +            (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)
  
positive.code:     [<TestCase>]
      let toString () : unit =
          // Test for empty substring.
          Substring ("Hello World!", 3, 0)
          |> Substring.toString
          |> should equal String.empty
  
          Substring ("Hello World!", 3, 6)
          |> Substring.toString
          |> should equal "lo Wor"
  
      [<TestCase>]
      let toArray () : unit =
          // Test for empty substring.
          Substring ("Hello World!", 3, 0)
          |> Substring.toArray
          |> should equal Array.empty
  
          Substring ("Hello World!", 3, 6)
          |> Substring.toArray
          |> should equal [| 'l'; 'o'; ' '; 'W'; 'o'; 'r'; |]
  
  
negative.code:         do
              let str = "The quick brown fox jumps over the lazy dog."
  
              let substr = substring (str, 4, 15)  // "quick brown fox"
              
              Substring.sub substr 6 5
              |> Substring.toString
  