The idea that the XML analogue to a primary key would become an attribute and everything else is elements. Hmm, wait. I guess it sort of depends.

Okay, here's what I really think. No, wait, that isn't it either. (Damn.) What I was thinking was that anything there can be only one of -- an atom can only have one atomic weight, one name -- is an attribute. Then the elements would be however many protons, neutrons and electrons. Each of those would have as elements their size, charge, etc., and would have as elements the number of quarks, gluons, yadda yadda. But that seems to sacrifice flexibility and usability for theoretical "correctness."

So, umm, like I said ... it depends. HTH.