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

[jpenny@universal-fasteners.com]

 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