code: func (sto *overlayStorage) RemoveBlobs(ctx context.Context, blobs []blob.Ref) error {
  	if sto.deleted == nil {
  		return blobserver.ErrNotImplemented
  	}
  
  	err := sto.upper.RemoveBlobs(ctx, blobs)
  	if err != nil {
  		return err
  	}
  
  	m := sto.deleted.BeginBatch()
  	for _, br := range blobs {
  		m.Set(br.String(), "1")
  	}
  	return sto.deleted.CommitBatch(m)
  }
docs: // RemoveBlobs marks the given blobs as deleted, and removes them if they are in the upper layer.