docs: def rollback(self):
          """Revert the previous modification to the refpkg.
          """
          # This is slightly complicated because of Python's freakish
          # assignment semantics and because we don't store multiple
          # copies of the log.
          if self.contents['rollback'] is None:
              raise ValueError("No operation to roll back on refpkg")
          future_msg = self.contents['log'][0]
          rolledback_log = self.contents['log'][1:]
          rollforward = copy.deepcopy(self.contents)
          rollforward.pop('rollback')
          self.contents = self.contents['rollback']
          self.contents['log'] = rolledback_log
          self.contents['rollforward'] = [future_msg, rollforward]
          self._sync_to_disk()