code: def _rollback(self, where):
          
  
          self.output.seek(where)
          self.output.truncate()
          keys_to_delete = []
          for k, v in self.compress.iteritems():
              if v >= where:
                  keys_to_delete.append(k)
          for k in keys_to_delete:
              del self.compress[k]
docs: Truncate the output buffer at offset I{where}, and remove any
          compression table entries that pointed beyond the truncation
          point.
  
          @param where: the offset
          @type where: int