Return the items with the higest score.

        If this ScoreSet is empty, returns None.

def highest(self):
        """Return the items with the higest score.

        If this ScoreSet is empty, returns None.
        """
        scores = self.scores()
        if not scores:
            return None
        maxscore = max(map(score, scores))
        return filter(lambda x: score(x) == maxscore, scores)