code: def add_event(self, key, event):
          
          if key in self.keys or key in self.events:
              raise EventStore.EventKeyAlreadyExistError(
                  "The key already existed: {0}".format(key))
          self.keys.append(key)
          # Important no exceptions happen between these lines!
          self.events[key] = event
docs: Add an event and its corresponding key to the store.