Deriving from DocBook schema

Hi,

I have to define my own special purpose XML schema in Relax NG.
Because the XML files need to have some documentation in it, I
like to allow some DocBook constructs, so it would be most
convenient just to derive from it. In DTD, I did - successfully
the following in my own DTD:

<!ENTITY % local.para.class "|docu">
<!ENTITY % sDocBookDTD PUBLIC "-//OASIS//DTD Simplified DocBook XML V1.0//EN"
 "http://www.oasis-open.org/docbook/xml/simple/1.0/sdocbook.dtd">
%sDocBookDTD;
...
<!ELEMENT myelem (%textobject.mix;)*>

How would I do that (or something with similar effect) using
Relax NG based DocBook? Thanks in advance!

Btw. Relax NG is much nicer than DTD :-)

Best regards,
W. Borgert