[luatex] writing to tex stream

[Frank Kuehnel]

Is there any documentation on how to write to a custom defined output-stream?

I couldn't find any help on how to set up an output stream defined in tex and 
used
in lua, i.e

\newwrite\xmlfile
\immediate\openout\xmlfile=test.xml
\immediate\write\xmlfile{<?xml version="1.0"?>}

\directlua{
        tex.print(... ???
}

Thanks for your help,

Frank