code: func CheckCertificateFormatFlag(s string) (string, error) {
  	switch s {
  	case teleport.CertificateFormatStandard, teleport.CertificateFormatOldSSH, teleport.CertificateFormatUnspecified:
  		return s, nil
  	default:
  		return "", trace.BadParameter("invalid certificate format parameter: %q", s)
  	}
  }
docs: // CheckCertificateFormatFlag checks if the certificate format is valid.