code: func (s accessSet) scopeParam() string {
  	scopes := make([]string, 0, len(s))
  
  	for resource, actionSet := range s {
  		actions := strings.Join(actionSet.keys(), ",")
  		scopes = append(scopes, fmt.Sprintf("%s:%s:%s", resource.Type, resource.Name, actions))
  	}
  
  	return strings.Join(scopes, " ")
  }
docs: // scopeParam returns a collection of scopes which can
  // be used for a WWW-Authenticate challenge parameter.
  // See https://tools.ietf.org/html/rfc6750#section-3