code: func New(opt Opt) cache.ExternalRefCheckerFunc {
  	return func() (cache.ExternalRefChecker, error) {
  		return &checker{opt: opt, layers: lchain{}, cache: map[string]bool{}}, nil
  	}
  }
docs: // New creates new image reference checker that can be used to see if a reference
  // is being used by any of the images in the image store