[dragonwriter]

> It is again a workaround because <authors> is a member of a collection - it is not a tuple attribute.

Its an element not an attribute, but the set of elements and the set of attributes are both collections.

> Which suggests that we cannot enforce this separation and an application has to understand itself which element is an object property and which element is a member of a collection

It's true that that is not part of bare XML and the nature of attributes vs. elements, even though you want to impose it there.

OTOH to the extent that those words correspond to a well-rounded semantic distinction, it is arguably captured in schema languages, and not mere application-level knowledge.