docs: def skip_format_url(format_, url):
      """
      Checks whether a give format/url should be skipped and not downloaded.
  
      @param format_: Filename format (extension).
      @type format_: str (e.g. html, txt, zip, pdf)
  
      @param url: URL.
      @type url: str
  
      @return: True if format/url should be skipped, False otherwise.
      @rtype bool
      """
      # Do not download empty formats
      if format_ == '':
          return True
  
      # Do not download email addresses
      if ('mailto:' in url) and ('@' in url):
          return True
  
      # Is this localhost?
      parsed = urlparse(url)
      if parsed.hostname == 'localhost':
          return True
  
      # These are trusted manually added formats, do not skip them
      if RE_VALID_FORMATS.match(format_):
          return False
  
      # Simple formats only contain letters, numbers, "_" and "-"
      # If this a non simple format?
      if RE_NON_SIMPLE_FORMAT.match(format_):
          return True
  
      # Is this a link to the site root?
      if parsed.path in ('', '/'):
          return True
  
      # Do not skip
      return False