xi:include problem

[Alex Kleider <akleider@sonic.net>]

I read in Norman Walsh's book:
"""
As time passes, the use of DTD-based mechanisms like entities is
diminishing. If you have an eye on the future, to the extent that it is
practical, it is probably better to use XInclude than entities.
"""
so have tried to switch from entities to XInclude but have hit a stone
wall!

Perhaps someone might be able to tell me what's missing in the way I'm
trying to do it.  Thanks in advance.  Alex

<article xmlns="http://docbook.org/ns/docbook"; version="5.0"
         xmlns:xlink="http://www.w3.org/1999/xlink";
         xmlns:xi="http://www.w3.org/2001/XInclude";
                                                            >

    <xi:include href="dp_info.xml"/>
    <xi:include href="dp_preface.xml"/>
    <xi:include href="new_plug.xml"/>
    <xi:include href="getting_in.xml"/>
    <xi:include href="minDual.xml"/>
    <xi:include href="card_prep.xml"/>
    <xi:include href="pathagar.xml"/>

</article>

<--  Error reported by
$ jing /usr/share/xml/docbook/schema/rng/5.0/docbook.rng dreamplug.xml
"""
/home/alex/Notes/Markup/Docbook/Opus/dreamplug.xml:6:33: error: element 
"xi:include" not allowed here; expected element "info", "subtitle", 
"title" or "titleabbrev"
/home/alex/Notes/Markup/Docbook/Opus/dreamplug.xml:7:36: error: element 
"xi:include" not allowed here; expected element "info", "subtitle", 
"title" or "titleabbrev"
/home/alex/Notes/Markup/Docbook/Opus/dreamplug.xml:8:34: error: element 
"xi:include" not allowed here; expected element "info", "subtitle", 
"title" or "titleabbrev"
/home/alex/Notes/Markup/Docbook/Opus/dreamplug.xml:9:36: error: element 
"xi:include" not allowed here; expected element "info", "subtitle", 
"title" or "titleabbrev"
/home/alex/Notes/Markup/Docbook/Opus/dreamplug.xml:10:33: error: element 
"xi:include" not allowed here; expected element "info", "subtitle", 
"title" or "titleabbrev"
/home/alex/Notes/Markup/Docbook/Opus/dreamplug.xml:11:35: error: element 
"xi:include" not allowed here; expected element "info", "subtitle", 
"title" or "titleabbrev"
/home/alex/Notes/Markup/Docbook/Opus/dreamplug.xml:12:34: error: element 
"xi:include" not allowed here; expected element "info", "subtitle", 
"title" or "titleabbrev"
/home/alex/Notes/Markup/Docbook/Opus/dreamplug.xml:22:11: error: element 
"article" incomplete; expected element "info", "subtitle", "title" or 
"titleabbrev"
-->