[Zope] authentication via form

Hi, I'm developing a small site in Zope with a simple, own management interface (where the contets of Documents can be easily changed using forms). The last few days' change in the ZopeBook on www.zope.org seems to be very useful, thanks for the authors! Although I could not find out, how to authenticate users not by calling a zope object that needs special permissions to access, but using a form with a login/passwd box and passing the arguments to some kind of functions. Can REQUEST.['AUTHENTICATED_USER'] be modified somehow? I could not find any function that I suppose to do this. Gergely Borgulya