code: func (l *tarexporter) releaseLayerReferences(imgDescr map[image.ID]*imageDescriptor) error {
  	for _, descr := range imgDescr {
  		if descr.layerRef != nil {
  			os := descr.image.OS
  			if os == "" {
  				os = runtime.GOOS
  			}
  			l.lss[os].Release(descr.layerRef)
  		}
  	}
  	return nil
  }
docs: // releaseLayerReferences will release all the image top layer references