I should have been more precise. I am using the treemenu and I wanted to know the parent of a node selected by this treemenu... But as far I as understood the documentation I don't think it can be done. Am I wrong?
The treemenu operator doesn't give you full nodes; see http://ez.no/doc/ez_publish/technical_manual/3_6/reference/template_operators/miscellaneous/treemenu
Among the things it does give you, however, is the IDs. So you can always fetch() each and every node. Can be quite a nightmare performance-wise of course.