DOCBOOK: Attribute 'xml:base' in the DocBook DTD

Hello, All!

I'm using new version of libxml2 (2.4.24) and XInclude for 
merging my documents. This version support attribute 'xml:base' 
but DocBook/XML DTD 4.2 is not. So, when I try to validate 
DocBook documents with
xmllint --xinclude --noout --postvalid test.xml

I get error:
No declaration for attribute base of element refentry
Document test.xml does not validate

Daniel Veillard says
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The xml:base is a requirement of XInclude
    http://www.w3.org/TR/xinclude/#base
And the DocBook DTD didn't got xml:base added as a possible 
attribute for all elements.
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It's possible to add this attribute ?

Sorry for bad English.

-- 
Regards, Vyt
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