How to specify null value and collection value as an XPATH function parameter value

Is it possible to specify a null value to an XPATH function as a 
parameter value?

Also, is it possible to specify a collection (e.g. collection of 
strings)  to an XPATH function as a parameter value?

In my case, its an extension function, so I suppose anything is possible 
but I am curious what are the rules.

Thanks.

-- 
Regards,
Farrukh Najmi

Web: http://www.wellfleetsoftware.com