RELAX NG pluggable type libraries

Do any RELAX NG implementations support user defined type libraries? 
i.e. something like XSLT extension functions?

For instance, is it possible in any tools (and if so which ones) to plug 
in my own code written in Java/C#/Python/XSLT/whatever that would verify 
that an element contains a prime number?

-- 
Elliotte Rusty Harold  
XML in a Nutshell 3rd Edition Just Published!
http://www.cafeconleche.org/books/xian3/
http://www.amazon.com/exec/obidos/ISBN=0596007647/cafeaulaitA/ref=nosim