code: def once(self, event, callback):
          'Define a callback to handle the first event emitted by the server'
          self._once_events.add(event)
          self.on(event, callbackf once(self, event, callback):
          'Define a callback to handle the first event emitted by the server'
          self._once_events.add(event)
          self.on(event, callback)
docs: Define a callback to handle the first event emitted by the server