include pretty printed program listings

is it possible to have external html-ized pretty printed code displayed 'as is' 
using the docbook xsl stylesheets?

something like:


   <programlisting condition="plain">
     <xi:include  href="foo.rb"  parse="text" />
   </programlisting>

   <!-- will be copied into the result tree without being escaped -->
   <programlisting condition="verypretty">
     <xi:include  href="foo.rb.html" />
   </programlisting>


I understand that the document will not be valid docbook anymore, but the 
included fragment is well formed.

thanks,
Markus