code: func UnwrapOpID(c context.Context, token string, caller identity.Identity) (int64, error) {
  	body, err := opToken.Validate(c, token, []byte(caller))
  	if err != nil {
  		return 0, errors.Annotate(err, "failed to validate upload operation ID token").Err()
  	}
  	id, err := strconv.ParseInt(body["id"], 10, 64)
  	if err != nil {
  		return 0, errors.Annotate(err, "invalid upload operation ID %q", body["id"]).Err()
  	}
  	return id, nil
  }
docs: // UnwrapOpID extracts upload operation ID from a HMAC-protected string.