[Nml-wg] XML syntax for NML relations

[Freek Dijkstra]

Maybe it helps to put my requirements for the "relation" XML syntax on
the table again, without the examples.

1. Be extensible
2. It should be possible to create a specific validator for each
relation type.
3. Parsers should be able to recognise an unknown relation type as a
relation subclass (rather then simply an unknown element)

Hope this helps a bit.

Freek