Prefixes in Trig

Hi,

I am writing RDF datasets out from Jena to disk and I noticed that the
prefix definitions are missing, meaning, all predicates are written out in
the long URI form rather than something compact like "foaf:knows" with the
foaf prefix defined up at the top. The name spaces were defined the a Jena
Model and then that model was added as a named graph to a empty dataset then
written out to disk.  Is this expected behavior from Jena or a bug or am I
missing something here?  - Erich