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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def save_fast_format(self, filename):
 """ Save a reach instance in a fast format. The reach fast format stores the words and vectors of a Reach instance separately in a JSON and numpy format, respectively. Parameters filename : str The prefix to add to the saved filename. Note that this is not the real filename under which these items are stored. The words and unk_index are stored under "{filename}_words.json", and the numpy matrix is saved under "{filename}_vectors.npy". """

        items, _ = zip(*sorted(self.items.items(), key=lambda x: x[1]))
        items = {"items": items,
                 "unk_index": self.unk_index,
                 "name": self.name}

        json.dump(items, open("{}_items.json".format(filename), 'w'))
        np.save(open("{}_vectors.npy".format(filename), 'wb'), self.vectors)