code: def get_fulltext_links(doi):
      
      metadata = get_metadata(doi)
      if metadata is None:
          return None
      links = metadata.get('link')
      return links
docs: Return a list of links to the full text of an article given its DOI.
      Each list entry is a dictionary with keys:
      - URL: the URL to the full text
      - content-type: e.g. text/xml or text/plain
      - content-version
      - intended-application: e.g. text-mining