code: func (k *Key) CertPrincipals() ([]string, error) {
  	publicKey, _, _, _, err := ssh.ParseAuthorizedKey(k.Cert)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  	cert, ok := publicKey.(*ssh.Certificate)
  	if !ok {
  		return nil, trace.BadParameter("no certificate found")
  	}
  	return cert.ValidPrincipals, nil
  }
docs: // CertPrincipals returns the principals listed on the SSH certificate.