[Zope] Change rendering of a folder

Hi, I would like to subclass the folder class and change the way it gets rendered. How would no how will I do that? Usually, a folder object looks if it contains a index_html and calls(renders) it if so. I would like to change this behavior and call a method of my own when the object gets rendered. I remember the rstx_document example, where you subclass from the original and add "Renderable" to the base classes(I think it was important which first), then provide a "render" method in your new ZClass and that's it. This didn't work for "Folder". btw, would I subclass from "OFS: Folder" or from "ObjectManager"? tia, Danny