code: def change_index(self, offset=0):
          
          src = {}
          src["box"] = self.__get_box_path()
          src["img"] = self.__get_img_path()
          src["thumb"] = self._get_thumb_path()
  
          page_nb = self.page_nb
  
          page_nb += offset
  
          logger.info("--> Moving page %d (+%d) to index %d"
                      % (self.page_nb, offset, page_nb))
  
          self.page_nb = page_nb
  
          dst = {}
          dst["box"] = self.__get_box_path()
          dst["img"] = self.__get_img_path()
          dst["thumb"] = self._get_thumb_path()
  
          for key in src.keys():
              if self.fs.exists(src[key]):
                  if self.fs.exists(dst[key]):
                      logger.error("Error: file already exists: %s" % dst[key])
                      assert(0)
                  self.fs.rename(src[key], dst[key])
docs: Move the page number by a given offset. Beware to not let any hole
          in the page numbers when doing this. Make sure also that the wanted
          number is available.
          Will also change the page number of the current object.