code: def doi_input(doi_string, download=True):
      
      log.debug('DOI Input - {0}'.format(doi_string))
      doi_string = doi_string[4:]
      if '10.1371' in doi_string:  # Corresponds to PLoS
          log.debug('DOI string shows PLoS')
          xml_url = plos_doi_to_xmlurl(doi_string)
      else:
          log.critical('DOI input for this publisher is not supported')
          sys.exit('This publisher is not yet supported by OpenAccess_EPUB')
      return url_input(xml_url, download)
docs: This method accepts a DOI string and attempts to download the appropriate
      xml file. If successful, it returns a path to that file. As with all URL
      input types, the success of this method depends on supporting per-publisher
      conventions and will fail on unsupported publishers