code: func signCertWithPrivateKey(pk string, certificate string) (ssh.AuthMethod, error) {
  	rawPk, err := ssh.ParseRawPrivateKey([]byte(pk))
  	if err != nil {
  		return nil, fmt.Errorf("failed to parse private key %q: %s", pk, err)
  	}
  
  	pcert, _, _, _, err := ssh.ParseAuthorizedKey([]byte(certificate))
  	if err != nil {
  		return nil, fmt.Errorf("failed to parse certificate %q: %s", certificate, err)
  	}
  
  	usigner, err := ssh.NewSignerFromKey(rawPk)
  	if err != nil {
  		return nil, fmt.Errorf("failed to create signer from raw private key %q: %s", rawPk, err)
  	}
  
  	ucertSigner, err := ssh.NewCertSigner(pcert.(*ssh.Certificate), usigner)
  	if err != nil {
  		return nil, fmt.Errorf("failed to create cert signer %q: %s", usigner, err)
  	}
  
  	return ssh.PublicKeys(ucertSigner), nil
  }
docs: // Create a Cert Signer and return ssh.AuthMethod