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

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