Node factories

If I have Concept with Child. Child has ordinal number. Concrete syntax is like this:
1.message 1
2.message 2
3.message 3

I want to implement in my language that ordinal number (step) is set automatically. It is not problem if I put new message at the end. I use node factories for this purpose, calculate the number of messages and increase for 1.

But, I have some problem if I put new message somewhere between 1 and 3 message.

node factories insertNewMessage         
   
node concept: Messages
   description : <none>           
   set-up : (sampleNode, newNode, model, enclosingNode)->void {

  node<Step> ss = new node<Step>();  

  node<Message> aa;   
  aa = (node<Message>) newNode;
  if (aa.isInstanceOf(Messages)) {
    // increase step
    ...
    newNode.step = ss;
  } else {

  }
  

  //update all messages number
  node<RootConcept> ms = (node<RootConcept>) enclosingNode;
  ms.updateSteps();
}

Updates work when I insert next message.

How can I get information where I want to insert new message, what is the message before? or
how to update message steps.