code: def look_for_hdl_urls(citation_elements):
      
      for el in citation_elements:
          if el['type'] == 'URL':
              match = re_hdl.match(el['url_string'])
              if match:
                  el['type'] = 'HDL'
                  el['hdl_id'] = match.group('hdl_id')
                  del el['url_desc']
                  del el['url_string']
docs: Looks for handle identifiers that have already been identified as urls
  
         When finding an hdl, creates a new HDL element.
         @param citation_elements: (list) elements to process