Extension functions in xsl-fo

Hello!

Are extension functions allowed in xsl-fo expressions? The Recommendation says 
nothing about it and taking into account the fact that FunctionName 
production[1] is NCName, one may conclude that extension functions (in form we 
all used to in XPath) are not allowed.
But on the other hand the Recommendation defines *Core* Function Library and 
one may conclude that noncore functions might exist too.
Please, shed some light upon the issue.

[1] http://www.w3.org/TR/xsl/slice5.html#NT-FunctionName
-- 
Oleg Tkachenko
eXperanto team
Multiconn Technologies, Israel