code: def collections(self):
          
          import time
          gotten_feed_num = 20
          offset = 0
          data = {
              'method':'next',
              '_xsrf': self.xsrf
          }
          while gotten_feed_num >= 10:
              data['params'] = "{\"answer_url\": %d,\"offset\": %d}" % (self.id, offset)
              res = self._session.post(url=Get_Collection_Url, data=data)
              gotten_feed_num = len(res.json()['msg'])
              offset += gotten_feed_num
              soup = BeautifulSoup(''.join(res.json()['msg']))
              for zm_item in soup.find_all('div', class_='zm-item'):
                  url = Zhihu_URL + zm_item.h2.a['href']
                  name = zm_item.h2.a.text
                  links = zm_item.div.find_all('a')
                  owner = Author(links[0]['href'], session=self._session)
                  follower_num = int(links[1].text.split()[0])
                  yield Collection(url, owner=owner, name=name,
                                   follower_num=follower_num,
                                   session=self._session)
  
              time.sleep(0.2)
docs: 获取包含该答案的收藏夹
  
          :return: 包含该答案的收藏夹
          :rtype: Collection.Iterable
  
          collect_num 未必等于 len(collections)，比如:
          https://www.zhihu.com/question/20064699/answer/13855720
          显示被收藏 38 次，但只有 30 个收藏夹