[lojban-beginners] cmavo can be selbri?

[Starling <pubsynx7hye@pacbell.net>]

Whee, don't know if anyone listens to this list, but definitely have a
few questions.  Here goes the first one...

I know that gismu can be selbri... seen enough examples of that.  But
the definition for 'du' calls it both cmavo and selbri.  Is this a
valid question in lojban:

xu la xu cu du la cmavo

If that is true, then can 'xu' itself be used as a selbri?  Or is it
just that some cmavo cannot be used as selbri, since they don't define
a predicate structure?  Or perhaps in the 'xu' example, it would be
the selbri for establishing a question regarding the sumti?

I understand that cmavo itself is a cmavo-establishment selbri, so it
would be better to ask instead

xu zo'e cmavo la xu

But I was curious as to whether the 'du' cmavo could be used as a selbri.