code: func checkIfReadyForAdditionalControlPlane(initConfiguration *kubeadmapi.ClusterConfiguration, hasCertificateKey bool) error {
  	// blocks if the cluster was created without a stable control plane endpoint
  	if initConfiguration.ControlPlaneEndpoint == "" {
  		return errors.New("unable to add a new control plane instance a cluster that doesn't have a stable controlPlaneEndpoint address")
  	}
  
  	if !hasCertificateKey {
  		// checks if the certificates that must be equal across controlplane instances are provided
  		if ret, err := certs.SharedCertificateExists(initConfiguration); !ret {
  			return err
  		}
  	}
  
  	return nil
  }
docs: // checkIfReadyForAdditionalControlPlane ensures that the cluster is in a state that supports
  // joining an additional control plane instance and if the node is ready to preflight