docs: def get_referer(req, replace_ampersands=False):
      """ Return the referring page of a request.
      Referer (wikipedia): Referer is a common misspelling of the word
      "referrer"; so common, in fact, that it made it into the official
      specification of HTTP. When visiting a webpage, the referer or
      referring page is the URL of the previous webpage from which a link was
      followed.
      @param req: request
      @param replace_ampersands: if 1, replace & by &amp; in url
                                 (correct HTML cannot contain & characters alone)
      """
      try:
          referer = req.headers_in['Referer']
          if replace_ampersands == 1:
              return referer.replace('&', '&amp;')
          return referer
      except KeyError:
          return ''