code: def _undo_save(self, datastreams, logMessage=None):
          
          return [ds for ds in datastreams if self.dscache[ds].undo_last_save(logMessage)]
docs: Takes a list of datastreams and a datetime, run undo save on all of them,
          and returns a list of the datastreams where the undo succeeded.
  
          :param datastreams: list of datastream ids (should be in self.dscache)
          :param logMessage: optional log message