code: func createImageExistenceMap(nodes []*v1.Node) map[string]sets.String {
  	imageExistenceMap := make(map[string]sets.String)
  	for _, node := range nodes {
  		for _, image := range node.Status.Images {
  			for _, name := range image.Names {
  				if _, ok := imageExistenceMap[name]; !ok {
  					imageExistenceMap[name] = sets.NewString(node.Name)
  				} else {
  					imageExistenceMap[name].Insert(node.Name)
  				}
  			}
  		}
  	}
  	return imageExistenceMap
  }
docs: // createImageExistenceMap returns a map recording on which nodes the images exist, keyed by the images' names.