Returns:
            dict: {cls_number: synset_id}

def get_synset_1000(self):
        """
        Returns:
            dict: {cls_number: synset_id}
        """
        fname = os.path.join(self.dir, 'synsets.txt')
        assert os.path.isfile(fname)
        lines = [x.strip() for x in open(fname).readlines()]
        return dict(enumerate(lines))