understanding why trang is upset

The exchange about preprocessing RNG to make it acceptable to trang to 
create DTDs
prompted me to revisit my own situation. Some careful staring reduced 
the problem to
a series of

   error: sorry, cannot handle this kind of "choice"

messages. But now I am stuck. I really do not understand what trang is 
objecting to
in things like this

       macro.paraContent =
          (text | class.phrase | class.inter | class.Incl)*

If any of you clever folk have a moment to spare, can you run trang
on http://users.ox.ac.uk/~rahtz/rahtz.rng and see if you can explain
the error messages?


Sebastian Rahtz