Zend_Session_Namespace -> navigation

Zend_Session_Namespace -> navigation

I'm new to Zend and only using it because I've got to update some other
guy's old code. Anyway, I have a Zend_Session_Namespace object. On which
navigation is accessed. Ie
$topNavSession = new Zend_Session_Namespace( 'topNav' );
$variable = $topNavSession->navigation;
So the question is, what is happening when navigation is called? How do I
edit what my navigation returns? It's not in the docs for the class, and
my IDE doesn't seem to find it in the inheritance.