code: func ParseBuilderID(s string) (*pb.BuilderID, error) {
  	parts := strings.Split(s, "/")
  	if len(parts) != 3 {
  		return nil, fmt.Errorf("invalid builder id; must have 2 slashes")
  	}
  	return &pb.BuilderID{
  		Project: parts[0],
  		Bucket:  parts[1],
  		Builder: parts[2],
  	}, nil
  }
docs: // ParseBuilderID parses a "{project}/{bucket}/{builder}" string.
  // Opposite of FormatBuilderID.