code: def to_url(arxiv_ids):
      
      if isinstance(arxiv_ids, list):
          return [ARXIV_URL.format(arxiv_id=arxiv_id) for arxiv_id in arxiv_ids]
      else:
          return ARXIV_URL.format(arxiv_id=arxiv_ids)
docs: Convert a list of canonical DOIs to a list of DOIs URLs.
  
      :param dois: List of canonical DOIs.
      :returns: A list of DOIs URLs.
  
      >>> to_url('1506.06690')
      'http://arxiv.org/abs/1506.06690'
  
      >>> to_url('1506.06690v1')
      'http://arxiv.org/abs/1506.06690v1'