code: func (defaultNodeIdentifier) NodeIdentity(u user.Info) (string, bool) {
  	// Make sure we're a node, and can parse the node name
  	if u == nil {
  		return "", false
  	}
  
  	userName := u.GetName()
  	if !strings.HasPrefix(userName, nodeUserNamePrefix) {
  		return "", false
  	}
  
  	isNode := false
  	for _, g := range u.GetGroups() {
  		if g == user.NodesGroup {
  			isNode = true
  			break
  		}
  	}
  	if !isNode {
  		return "", false
  	}
  
  	nodeName := strings.TrimPrefix(userName, nodeUserNamePrefix)
  	return nodeName, true
  }
docs: // NodeIdentity returns isNode=true if the user groups contain the system:nodes
  // group and the user name matches the format system:node:<nodeName>, and
  // populates nodeName if isNode is true