MathML in docbook 4.3 errors

Hi all,
   We're trying to use mathml for some equations in some of our xml docs 
but we keep getting 'failed to load external entities' errors. 
mathmldb.dtd is installed locally but it doesn't seem to matter.

Is it not possible to embed mathml within a docbook (book or article) 
file?  Does it need to be its own file and then xincluded?

The doctype def is:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE book
      PUBLIC "-//OASIS//DTD DocBook MathML Module V1.0//EN"
 
"http://www.oasis-open.org/docbook/xml/mathml/1.0/dbmathml.dtd">

      <book id="xkbproto">
...


<informalequation>
  <mml:math>
   <mml:msup>
    <mml:mfenced>
     <mml:mi>a</mml:mi>
     <mml:mo>+</mml:mo>
     <mml:mi>b</mml:mi>
    </mml:mfenced>
    <mml:mn>2</mml:mn>
   </mml:msup>
  </mml:math>
</informalequation>


... which caused the following validation errors:

$ make
    Making all in specs
    make[1]: Entering directory 
`/home/fcarrijo/softwares/repositorio/xorg/proto/kbproto/specs'
      GEN    xkbproto.html
     xmlto: 
/home/fcarrijo/softwares/repositorio/xorg/proto/kbproto/specs/xkbproto.xml 
does not validate (status 3)
     xmlto: Fix document syntax or use --skip-validation option
     I/O error : Attempt to load network entity 
http://www.oasis-open.org/docbook/xml/mathml/1.0/dbmathml.dtd
 
/home/fcarrijo/softwares/repositorio/xorg/proto/kbproto/specs/xkbproto.xml:4: 
warning: failed to load external entity 
"http://www.oasis-open.org/docbook/xml/mathml/1.0/dbmathml.dtd"
 
"http://www.oasis-open.org/docbook/xml/mathml/1.0/dbmathml.dtd">
                                                                           ^
 
/home/fcarrijo/softwares/repositorio/xorg/proto/kbproto/specs/ch04.xml:417: 
namespace error : Namespace prefix mml on math is not defined
     <mml:math>
              ^

Notes in libxml2 site point that "failed to load external entity" refers
to a misconfigured XML Catalog, which I tried to fix installing two other
packages...

     docbook-mathml
     w3-dtd-mathml

... and setting XML_DEBUG_CATALOG=1 before building, according to [1].
The environment variable set causes the build to output many more lines,
being these the most relevant:

     Trying system delegate file:///etc/xml/docbook-xml.xml
     Resolve URI 
http://www.oasis-open.org/docbook/xml/mathml/1.0/dbmathml.dtd
     I/O error : Attempt to load network entity 
http://www.oasis-open.org/docbook/xml/mathml/1.0/dbmathml.dtd
     warning: failed to load external entity 
"http://www.oasis-open.org/docbook/xml/mathml/1.0/dbmathml.dtd"
     validity error : Could not load the external subset 
"http://www.oasis-open.org/docbook/xml/mathml/1.0/dbmathml.dtd"
     Document 
/home/fcarrijo/softwares/repositorio/xorg/proto/kbproto/specs/xkbproto.xml 
does not validate
     Catalogs cleanup



Do you guys have any suggestions?

[0] http://www.docbook.org/specs/wd-docbook-mathml-1.1CR1.html
[1] http://xmlsoft.org/catalog.html#validate


thanks,
Matt