code: def subscribe(self, event, bet_ids):
          '''Subscribe to event for given bet ids.'''
          if not self._subscriptions.get(event):
              self._subscriptions[event] = set()
          self._subscriptions[event] = self._subscriptions[event].union(bet_idsf subscribe(self, event, bet_ids):
          '''Subscribe to event for given bet ids.'''
          if not self._subscriptions.get(event):
              self._subscriptions[event] = set()
          self._subscriptions[event] = self._subscriptions[event].union(bet_ids)
docs: Subscribe to event for given bet ids.