DOCBOOK-APPS: XInclude doesn't validate with xmllint

[John Himpel <jwhimpel at tseinc dot com>]

Greetings,

I am trying to validate two xml files using the modular document
technique that Bob Stayton shows in www.sagehill.net.

My version of xmllint:
xmllint: using libxml version 20423

File: Caller.xml
<?xml version='1.0' encoding='utf-8' ?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
                     
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"; >
<book id="XFree86">
<title>XFree86 Documentation (Docbook Version)</title>
<xi xmlns:xi="http://www.w3.org/2001/XInclude"; href="Called.xml" />
</book>

File: Called.xml

<?xml version="1.0"?>
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
                        
"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd"; >

<chapter id='Called'>
  <title>
       Chapter Title.
  </title>

  <section>
    <title>Preface</title>
    <para>Simple paragraph.</para>
  </section>
</chapter>

Command:
xmllint --catalogs --noout --xinclude  --valid --dtdvalid
http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd Caller.xml

Result:
Variable $SGML_CATALOG_FILES not set
Caller.xml:6: validity error: No declaration for attribute href on
element xi
<xi xmlns:xi="http://www.w3.org/2001/XInclude"; href="Called.xml" />
                                                                 ^
Caller.xml:6: validity error: No declaration for element xi
<xi xmlns:xi="http://www.w3.org/2001/XInclude"; href="Called.xml" />
                                                                  ^
Caller.xml:7: validity error: Element book content doesn't follow the
DTD
Expecting ((title , subtitle? , titleabbrev?)? , bookinfo? , (dedication
| toc | lot | glossary | bibliography | preface | chapter | reference |
part | article | appendix | index | setindex | colophon)*), got (title
xi )
</book>
      ^
Element book content doesn't follow the DTD

I don't know if the problem is DocBook or if the problem is xmllint.

Any pointers or suggestions would be welcome.

Thanks