Can anybody give me a little newbie help with calling a function
stored in overlay.js on pressing a button?

I have the following hello world example in which the function is
called when a menu item is selected. I'm a bit stuck because I don't
understand how the example works. Here it is:

overlay.xul


type="text/css"?>

xmlns="http://www.mozilla.org/keymaster/gatekeeper/
there.is.only.xul">