> Thanks! That would seem to solve the problem from the spec's
> perspective. As far as I can tell, the syntax is not supported in eXist
> (as of 2.0 preview r16247).

That's because I must have overlooked the parenthesized sequence type rule when reading the spec ;-) It is only mentioned in one short sentence in the text, so it never came to my attention. Thanks for pointing this out.