code: def get_linked_version(doi):
      
      try:
          request = requests.head(to_url(doi))
          return request.headers.get("location")
      except RequestException:
          return None
docs: Get the original link behind the DOI.
  
      :param doi: A canonical DOI.
      :returns: The canonical URL behind the DOI, or ``None``.
  
      >>> get_linked_version('10.1209/0295-5075/111/40005')
      'http://stacks.iop.org/0295-5075/111/i=4/a=40005?key=crossref.9ad851948a976ecdf216d4929b0b6f01'