anchor.diff:  let write_hints (_: string) (_: hints_db): unit =
     failwith "[record_hints_json]: not implemented"
   
  -let read_hints (_: string): option<hints_db> =
  -  failwith "[record_hints_json]: not implemented"
  +let read_hints (filename : string): option<hints_db> =
  +    if not (File.Exists filename) then
  +        None
  +    else
  +        failwith "[record_hints_json]: not implemented"
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:   | Inl of 'a
    | Inr of 'b
  
  let is_left = function
    | Inl _ -> true
    | _ -> false
  let is_right = function
  