DOCBOOK-APPS: Docbook XML - including files

This isn't a docbook-per se question..

Current I have a root document that has external entity references for a
bunch of articles:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
"docbookx.dtd" [
<!ENTITY foo SYSTEM "foo.xml">
<!ENTITY bar SYSTEM "bar.xml">
]>
<book id="test_book" >
<title>test book</title>
&foo;
&bar;
</book>

where foo.xml looks something like
<article>
...
</article>

what I'd _like_ to do is to have foo include a set of separate files
(say oen for each section of the article foo).

However using <!ENTITY ..> in the beginning of foo.xml spits parsing
errors... any hints on this one?

Rob