code: def format_arxiv_id(arxiv_id):
      
      if arxiv_id and "/" not in arxiv_id and "arXiv" not in arxiv_id:
          return "arXiv:%s" % (arxiv_id,)
      elif arxiv_id and '.' not in arxiv_id and arxiv_id.lower().startswith('arxiv:'):
          return arxiv_id[6:]  # strip away arxiv: for old identifiers
      else:
          return arxiv_id
docs: Properly format arXiv IDs.