[Zope] how can I determine the list of user names?

I would like to construct a pull down of the users defined in an acl_users of a particular location. Say in particular, that /protected/acl_users exists. How do I find (in dtml or in python), the list of names defined in that particular acl_users. To slightly generalize, how would one tree-walk to find the list of all acl_users above a particular object? Thanks Jim