code: def on(cls, *, event: str, callback: Callable):
          
          if isinstance(callback, list):
              for cb in callback:
                  cls._validate_callback(cb)
              previous_callbacks = cls._callbacks[event]
              cls._callbacks[event] = list(set(previous_callbacks + callback))
          else:
              cls._validate_callback(callback)
              cls._callbacks[event].append(callback)
docs: Stores and links the callback(s) to the event.
  
          Args:
              event (str): A string that specifies a Slack or websocket event.
                  e.g. 'channel_joined' or 'open'
              callback (Callable): Any object or a list of objects that can be called.
                  e.g. <function say_hello at 0x101234567> or
                  [<function say_hello at 0x10123>,<function say_bye at 0x10456>]
  
          Raises:
              SlackClientError: The specified callback is not callable.
              SlackClientError: The callback must accept keyword arguments (**kwargs).