anchor.diff:    | Inl of 'a
     | Inr of 'b
   
  -let is_left = function 
  +let is_left = function
     | Inl _ -> true
     | _ -> false
   let is_right = function
  
positive.code: type time = System.DateTime
  let now () = System.DateTime.Now
  let time_diff (t1:time) (t2:time) : float * int =
      let ts = t2 - t1 in 
      ts.TotalSeconds, int32 ts.TotalMilliseconds
  let record_time f = 
      let start = now () in
      let res = f () in 
      let _, elapsed = time_diff start (now()) in
      res, elapsed
  let get_file_last_modification_time f = System.IO.File.GetLastWriteTime f
  
negative.code:   e.Message
  
  let format_md5 bytes =
    let sb =
      Array.fold
        (fun (acc:StringBuilder) (by:byte) ->
          acc.Append(by.ToString("x2")))
        (new StringBuilder())
  