code: def add_handler(self, handler: Handler, group: int = 0):
          
          if isinstance(handler, DisconnectHandler):
              self.disconnect_handler = handler.callback
          else:
              self.dispatcher.add_handler(handler, group)
  
          return handler, group
docs: Use this method to register an update handler.
  
          You can register multiple handlers, but at most one handler within a group
          will be used for a single update. To handle the same update more than once, register
          your handler using a different group id (lower group id == higher priority).
  
          Args:
              handler (``Handler``):
                  The handler to be registered.
  
              group (``int``, *optional*):
                  The group identifier, defaults to 0.
  
          Returns:
              A tuple of (handler, group)