anchor.diff:  let handleable = function
     | Error _
     | NYI _
  +  | Stop
     | Err _ -> true
     | _ -> false
   
   let stop_if_err () =
       if get_err_count () > 0
  -    then raise (Err "Stopping due to errors")
  -    else ()
  +    then raise Stop
  
positive.code:     | _ -> None
  
  let err_exn exn =
      match issue_of_exn exn with
      | Some issue -> add_one issue
      | None -> raise exn
  
negative.code: 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
  