docs: def _add_url_rule(url_or_urls):
      """Register URL rule to application URL map."""
      old = current_app._got_first_request
      # This is bit of cheating to overcome @flask.app.setupmethod decorator.
      current_app._got_first_request = False
      if isinstance(url_or_urls, six.string_types):
          url_or_urls = [url_or_urls]
      map(lambda url: current_app.add_url_rule(url, 'invenio_pages.view', view),
          url_or_urls)
      current_app._got_first_request = old