code: func NewImageStore(fs StoreBackend, lss map[string]LayerGetReleaser) (Store, error) {
  	is := &store{
  		lss:       lss,
  		images:    make(map[ID]*imageMeta),
  		fs:        fs,
  		digestSet: digestset.NewSet(),
  	}
  
  	// load all current images and retain layers
  	if err := is.restore(); err != nil {
  		return nil, err
  	}
  
  	return is, nil
  }
docs: // NewImageStore returns new store object for given set of layer stores