code: func (cs Set) Ref() string {
  	domain, _, ref := cs.Split()
  	if domain == "projects" {
  		return ref
  	}
  	return ""
  }
docs: // Ref returns a ref component of a project-rooted config set or empty string
  // for all other sets.
  //
  // Use ProjectAndRef if you need to get both the project name and the ref.