[Zope] How can I check a user_acl folder to see if a user already exists?

[Rob Foster]

Here's the setup:

I'm trying to add users to my site by having them fill out a form with 
the following fields:

Username
Password
Confirm (password confirmation)

I want the form to post to a python script that will check the folder 
and do a redirect to another document depending on what happens... I 
don't know Python that well. I've tried doing it in dtml with no results.

Can someone help me with a little example code or at least point me to a 
product that does this? Thanks in advance. -Rob