code: def tell(self):
          
          if self._shifts:
              t = self._file.tell()
              if t == self._shifts[0]:
                  return 0
              elif t == self._shifts[-1]:
                  return len(self._shifts) - 1
              elif t in self._shifts:
                  return bisect_left(self._shifts, t)
              else:
                  return bisect_left(self._shifts, t) - 1
          raise self._implement_error
docs: :return: number of records processed from the original file