type priority = [
  | `Emerg    (* system is unusable *)
  | `Alert    (* action must be taken immediately *)
  | `Crit     (* critical conditions *)
  | `Err      (* error conditions *)
  | `Warning  (* warning conditions *)
  | `Notice   (* normal, but significant, condition *)
  | `Info     (* informational message *)
  | `Debug    (* debug-level message *)
]

  (** @param level minimum level of priority that will be reported, msg with
  priority less than this will be ignored *)
class sysLogger:
  ?level: priority ->
  unit ->
    object
        (** enable logging, by default logging is disabled *)
      method enable: unit
        (** disable logging *)
      method disable: unit
        (** log a message *)
      method log: priority -> string -> unit
    end

class processingLogger:
  unit ->
    object
      method log: string -> unit
      method asText: string
      method asHtml: string
    end