Beyond the class system, TEI elements have to be defined. How? (This is also known as the Durand Conundrum)

1. continue (as in P4) to use `raw' XML DTD language
2. maintain in DTD language but transform to some other schema language at the point of delivery
3. transform to some other schema language for maintenance and delivery
4. invent an entirely new abstract language for later transformation to some schema language

We chose a combination of 3 and 4 — revise our abstract language to use RelaxNG for content modelling (only).