Constraints on the elements in a nested XML tree?

Hi Folks,

I am dealing with XML trees.

No id/idrefs. No mixed content. 

Just plain old properly nested elements.

Suppose I have a pile of elements and I want a machine to assemble them into a tree of properly nested elements. The machine doesn't know the rules for assembly so I must tell it the constraints on the elements; in particular, the constraints on how they may be nested. 

Here's the list of constraints that I've come up with thus far:

1. An element cannot be a descendent of itself, i.e., no loops in the tree.

2. If elements e and e' are children of element E, then e cannot be a child of e' or vice versa.

What other constraints are there?

/Roger