docs: def _keepVol(self, vol):
          """ Mark this volume to be kept in path. """
          if vol is None:
              return
  
          if vol in self.extraVolumes:
              del self.extraVolumes[vol]
              return
  
          if vol not in self.paths:
              raise Exception("%s not in %s" % (vol, self))
  
          paths = [os.path.basename(path) for path in self.paths[vol]]
          newPath = self.selectReceivePath(paths)
  
          if self._skipDryRun(logger, 'INFO')("Copy %s to %s", vol, newPath):
              return
  
          self.butterVolumes[vol.uuid].copy(newPath)