docs: def keep(self, diff):
          """ Mark this diff (or volume) to be kept in path. """
          path = self.extraKeys[diff]
  
          if not path.startswith("/"):
              logger.debug("Keeping %s", path)
              del self.extraKeys[diff]
              return
  
          # Copy into self.userPath, if not there already
  
          keyName = self._keyName(diff.toUUID, diff.fromUUID, path)
          newPath = os.path.join(self.userPath, os.path.basename(path))
          newName = self._keyName(diff.toUUID, diff.fromUUID, newPath)
  
          if not self._skipDryRun(logger)("Copy %s to %s", keyName, newName):
              self.bucket.copy_key(newName, self.bucket.name, keyName)