code: def add(self, msg, callback, override=False):
          
          if override:
              if isinstance(callback, list):
                  self._dict[msg] = callback
              else:
                  self._dict[msg] = [callback]
          else:
              cb = self[msg]
              cb.append(callback)
              self._dict[msg] = cb
docs: Add a callback to the callback list.
  
          msg: Message template.
          callback: Callback method
          override: True - replace all existing callbacks for that template
                    False - append the list of callbacks for that message
                    Default is False