code: def check_unique_tags(tag_list):
    
    frozen_tags_seen = set()
    for tags in tag_list:
      frozen_tags = frozenset(tags)
      if frozen_tags in frozen_tags_seen:
        raise ValueError("Tags %r used repeatedly" % tags)
      frozen_tags_seen.add(frozen_tags)
docs: Checks that tag list contains each set of tags only once.