code: func (id BuildID) Validate() error {
  	switch {
  	case id.Master == "":
  		return errors.New("master is unspecified", grpcutil.InvalidArgumentTag)
  	case id.Builder == "":
  		return errors.New("builder is unspecified", grpcutil.InvalidArgumentTag)
  	case id.Number < 0:
  		return errors.New("nunber must be >= 0", grpcutil.InvalidArgumentTag)
  	default:
  		return nil
  	}
  }
docs: // Validate returns an error if id is invalid.