code: func (s *storage) deleteZipPack(ctx context.Context, br blob.Ref) error {
  	inUse, err := s.zipPartsInUse(ctx, br)
  	if err != nil {
  		return err
  	}
  	if len(inUse) > 0 {
  		return fmt.Errorf("can't delete zip pack %v: %d parts in use: %v", br, len(inUse), inUse)
  	}
  	if err := s.large.RemoveBlobs(ctx, []blob.Ref{br}); err != nil {
  		return err
  	}
  	return s.meta.Delete("d:" + br.String())
  }
docs: // deleteZipPack deletes the zip pack file br, but only if that zip
  // file's parts are deleted already from the meta index.