If a baby has been fed with breast milk, it's harder for the father to cradle them. All we can do is to wait them crying themselves to sleep. I dare say every baby should realise this inevitable fact.

Now I studied a bit about the formal way of processing XML trees. I have a vague intuition that the approach I've been practically using in my projects, which I developed without any theoretical background, has partly equivalent with the well-defined formal method. If it's the case, it might be a kind of help for someone who has the same problem as the one I had.