code: def add_subscription(self, channel, callback_function):
          
          if channel not in CHANNELS:
              CHANNELS.append(channel)
              SUBSCRIPTIONS[channel] = [callback_function]
          else:
              SUBSCRIPTIONS[channel].append(callback_function)
          # If a channel gets added after subscription has already been called
          # call subscribe on the individual channel, here.
          if self._subscribed:
              _LOGGER.info("New channel added after main subscribe call.")
              self._pubnub.subscribe().channels(channel).execute()
docs: Add a channel to subscribe to and a callback function to
          run when the channel receives an update.
          If channel already exists, create a new "subscription"
          and append another callback function.
  
          Args:
              channel (str): The channel to add a subscription too.
              callback_function (func): The function to run on an
                  update to the passed in channel.