anchor.diff:  exception Err of string
   exception Error of string * Range.range
   exception Warning of string * Range.range
  +exception Stop
   
   (* Raised when an empty fragment is parsed *)
   exception Empty_frag
  
positive.code:     | _ -> None
  
  let err_exn exn =
      match issue_of_exn exn with
      | Some issue -> add_one issue
      | None -> raise exn
  
negative.code: let handleable = function
    | Error _
    | NYI _
    | Stop
    | Err _ -> true
    | _ -> false
  
  let stop_if_err () =
      if get_err_count () > 0
      then raise Stop
  