Fetch a set of items with IDs greater than current set.

def prev(self):
        """Fetch a set of items with IDs greater than current set."""
        if self.limit and self.limit == self.num_tweets:
            raise StopIteration

        self.index -= 1
        if self.index < 0:
            # There's no way to fetch a set of tweets directly 'above' the
            # current set
            raise StopIteration

        data = self.results[self.index]
        self.max_id = self.model_results[self.index].max_id
        self.num_tweets += 1
        return data