[Zope] Setting a session's domain

Hi all, I'm developing a site that has multiple subdomains (i.e.: http://atlanta.website.com, http://nyc.website.com, etc.). Can anybody point me how can I get the session to be shared across these different subdomains? Unless someone can give me an opposite advise, I think I need to specify a "domain" to the cookie which is trivial for regular cookies but I just can't find how to do it with sessions (notice that I am I'm using Zope's regular request.SESSION). Thanks in advance, Pablo