code: def _sortAttributeValue(self, offset):
          
          if self._currentResults:
              pageStart = (self._currentResults[offset][
                  self.currentSortColumn.attributeID],
                           self._currentResults[offset][
                      '__item__'].storeID)
          else:
              pageStart = None
          return pageStart
docs: return the value of the sort attribute for the item at
          'offset' in the results of the last query, otherwise None.