Testing for ancestors

Hi All;

I am pretty new to XSL so I apologize if this is a trivial question.

If you have a specific node somewhere in the tree, and you are stepping
through the tree starting from the root. What is the best way to determine
if a node is an ancestor of the selected one?

I am trying to write a recursive function that generates an html menu (based
on specific nodes in an xml document) that gives the user the illusion of
expanding and collapsing as they select various nodes in the menu.

Thanks,

Luke