Get comic strips for an URL. If maxstrips is a positive number, stop after
        retrieving the given number of strips.

def getStripsFor(self, url, maxstrips):
        """Get comic strips for an URL. If maxstrips is a positive number, stop after
        retrieving the given number of strips."""
        self.hitFirstStripUrl = False
        seen_urls = set()
        while url:
            out.info(u'Get strip URL %s' % url, level=1)
            data = self.getPage(url)
            if self.shouldSkipUrl(url, data):
                out.info(u'Skipping URL %s' % url)
                self.skippedUrls.add(url)
            else:
                try:
                    yield self.getComicStrip(url, data)
                except ValueError as msg:
                    # image not found
                    out.exception(msg)
            if self.firstStripUrl == url:
                out.debug(u"Stop at first URL %s" % url)
                self.hitFirstStripUrl = True
                break
            if maxstrips is not None:
                maxstrips -= 1
                if maxstrips <= 0:
                    break
            prevUrl = self.getPrevUrl(url, data)
            seen_urls.add(url)
            if prevUrl in seen_urls:
                # avoid recursive URL loops
                out.warn(u"Already seen previous URL %r" % prevUrl)
                break
            url = prevUrl
            if url:
                # wait up to 2 seconds for next URL
                time.sleep(1.0 + random.random())