[Zope] CMF Member capitalization and duplication prevention (a mouth full)

[BZ]

I have been digging through the code trying to figure out how I can stop
members from registering with the same name but different capitalization
(ie, 'BZ', 'bz', 'Bz', 'bZ').

The closest I came was in"

http://cvs.zope.org/CMF/CMFDefault/RegistrationTool.py

Where there is a function line: if not self.isMemberIdAllowed(username):

I would like to try and edit that function but cannot find where it is in
the source code.

Has anyone done this and am I looking in the right area?

Thanks,
BZ