code: func (c *CertAuthID) Check() error {
  	if err := c.Type.Check(); err != nil {
  		return trace.Wrap(err)
  	}
  	if strings.TrimSpace(c.DomainName) == "" {
  		return trace.BadParameter("identity validation error: empty domain name")
  	}
  	return nil
  }
docs: // Check returns error if any of the id parameters are bad, nil otherwise