Thanks for the quick answer, is there any example on how to do this that you 
wrute below?

> One thing that's undocumented on the libxslt web site is that recent
> versions support extensions as 'plugins' in a 'libxslt-plugins'
> directory as an alternative to the previous requirement to make your own
> xsltproc-variant that linked to your own extension functions.

It's sound much more easy to do.

Thanks.

regards

Giulio