DOM Children & Events

I'm having some difficulties with a menu I'm making. I build up the
menu through DOM. I append childnodes to a tree. 2 types of children
are possible: url (a hyperlink) and sub (a submap).

The tree is thus of a structure

main
|-url1
|-url2
|-submap
|--url21
|--url22

To open up the submaps I set the onclick event to a function I
created:

itemnode.onclick=function() { openMap(itemnode) }

The problem now is that 1 click in a submap results in several times
the openmap function. A click on the url21 node is in fact a click on
a child of submap so the function in the onclick event of this submap
is called. But it is also a click on a child of main, so this onclick
function is also called.

How can you limit that the onclick event handler is only called on the
1 node (ic. submap) where you actually clicked on submap instead of
its urls?

Full code listing below....