combining schemas

I'm interested in experimenting with mixing docbook with a separate 
namespaced metadata schema, but I seem to be stuck with a "duplicate 
definition of start" error that I don't know how to get rid of.

Basically I want an instance like so:

<document>
    <meta>
       <mods:name/>
       <mods:titleInfo/>
    </meta>
    <content>
       <section>
          <title>Introduction</title>
          <para>blah, blah, blah</para>
       </section>
    </content>
</document>

You could imagine Dublin Core in place of MODS.

Here's what I have at the moment:
===============================
start = document
include "docbook.rnc"
include "mods3.rnc"
document = element document { meta, content }
meta = ModsSchema
content = { "article" | "book" | "chapter" }
===============================

What am I doing wrong??

Bruce