[Zope] dtml-tree and python

Hi! I need to create a dtml-tree which obtains the branches from a python function. The best doc (well, the only one) I found is http://www.zope.org/Members/anthony/tree-coding-tricks , but I can't get to work the very same example the author writes there. I have tried two ways: a) writing his python code in a python script, but it says I can't define a class in a python script; b) writing his python code in an external method, but it keeps asking me for authentication and eventually says I have no authorization. Can anyone tell me what the problem is? Has anyone succeeded in doing this, that is to say, building a dtml-tree that obtains the branches from a python method? Thanks in advance, Igor Leturia