[Zope] Limiting depth on object creation

Zopisitas, I am building a threaded discussion product which integrates with a news article product I recently wrote. Inside of the article (folderish Zclass) I would like to allow my users to post "reponses." Where I am running into problems is that I would like to limit my "reply-to" depth to only three levels. In other words, I would like to limit a user to only adding three subdirectories in a directory, or 3 folderish objects (responses) beneath a single folderish object (discussion folder). Is there an easy way to check for this? How does one accomplish this in zope? Thanks in advance, -Darin Lee