code: func (sto *overlayStorage) ReceiveBlob(ctx context.Context, br blob.Ref, src io.Reader) (sb blob.SizedRef, err error) {
  	sb, err = sto.upper.ReceiveBlob(ctx, br, src)
  	if err == nil && sto.deleted != nil {
  		err = sto.deleted.Delete(br.String())
  	}
  	return sb, err
  }
docs: // ReceiveBlob stores received blobs on the upper layer.