code: func previous(ctx context.Context, client *containerd.Client, c *containers.Container) (*revision, error) {
  	service := client.SnapshotService(c.Snapshotter)
  	sInfo, err := service.Stat(ctx, c.SnapshotKey)
  	if err != nil {
  		return nil, err
  	}
  	key := sInfo.Labels[previousRevision]
  	if key == "" {
  		return nil, ErrNoPreviousRevision
  	}
  	parts := strings.Split(key, ".")
  	timestamp, err := time.Parse(timestampFormat, parts[len(parts)-1])
  	if err != nil {
  		return nil, err
  	}
  	return &revision{
  		Timestamp: timestamp,
  		Key:       key,
  	}, nil
  }
docs: // nolint: interfacer