code: func NumericIDs(id string) (Resolver, error) {
  	if id == "root" {
  		return numericIDs{0}, nil
  	}
  
  	if i, err := strconv.Atoi(id); err == nil {
  		return numericIDs{i}, nil
  	}
  
  	return nil, fmt.Errorf("invalid id %q", id)
  }
docs: // NumericIDs returns a resolver that will resolve constant UID/GID values.
  // If the given id equals to "root" the resolver always resolves UID=0 and GID=0.
  // If the given id is a numeric literal i it always resolves UID=i and GID=i.
  // If the given id is neither "root" nor a numeric literal an error is returned.