def append(self, event, category=None):
        """
        Adds a new event to the trace store.
        The event may hava a category

        Args:
          event (spade.message.Message): the event to be stored
          category (str, optional): a category to classify the event (Default value = None)

        """
        date = datetime.datetime.now()
        self.store.insert(0, (date, event, category))
        if len(self.store) > self.size:
            del self.store[-1]