code: def getPrevUrl(self, url, data):
          
          if 'chapter04' in url:
              return url.replace('chapter04', 'chapter03')
          if 'chapter03' in url:
              return url.replace('chapter03', 'chapter02')
          if 'chapter02' in url:
              return url.replace('chapter02', 'chapter01')
          return None
docs: Find previous URL.