How to use the sacremoses.util.pairwise function in sacremoses

def get_pair_statistics(self):
        """Count frequency of all symbol pairs, and create index"""
        # Data structure of pair frequencies
        stats = Counter()
        # Index from pairs to words
        indices = defaultdict(lambda: Counter())

        for i, (word, freq) in enumerate(self.vocab):
            for prev, curr in pairwise(word):
                stats[prev, curr] += freq
                indices[prev, curr][i] += 1

        return stats, indices