code: def getPrevUrl(self, url, data):
          
          prevUrl = None
          if self.prevSearch:
              try:
                  prevUrl = self.fetchUrl(url, data, self.prevSearch)
              except ValueError as msg:
                  # assume there is no previous URL, but print a warning
                  out.warn(u"%s Assuming no previous comic strips exist." % msg)
              else:
                  prevUrl = self.prevUrlModifier(prevUrl)
                  out.debug(u"Found previous URL %s" % prevUrl)
                  getHandler().comicPageLink(self.getName(), url, prevUrl)
          return prevUrl
docs: Find previous URL.