text: """Utility functions for feed-generating code."""
  from h._compat import urlparse
  
  # See RFC4151 for details of the use and format of the tag date:
  #
  #   https://tools.ietf.org/html/rfc4151#section-2.1
  FEED_TAG_DATE='2015-09'
  
  
  def tag_uri_for_annotation(annotation, annotation_url):
      """Return a tag URI (unique identifier) for the given annotation.
  
      Suitable for use as the value of the <id> element of an <entry> in an
      Atom feed, or the <guid> element of an <item> in an RSS feed.
  
      :returns: A tag URI (RFC 4151) for use as the ID for the Atom entry.
      :rtype: string
  
      """
      domain = urlparse.urlparse(annotation_url(annotation)).hostname
      return u"tag:{domain},{date}:{id_}".format(domain=domain,
                                                 date=FEED_TAG_DATE,
                                                 id_=annotation["id"])
  