[Zope] Zope product and dtml-tree

[Rowan Woodhouse]

Hi,

I'm developing a Zope product that uses a dtml file to generate the html 
page. I'd like to include a dtml-tree based tree for navigation on the 
page. The custom object inherits from SimpleItem. When I include the 
follow code in the dtml file:

<dtml-tree>
   <dtml-var id>
</dtml-tree>

I get the following exception:

An error was encountered while publishing this resource.

exceptions.KeyError
Sorry, a site error occurred.

Traceback (innermost last):

     * Module ZPublisher.Publish, line 202, in publish_module_standard
     * Module ZPublisher.Publish, line 150, in publish
     * Module ZPublisher.Publish, line 119, in publish
     * Module ZPublisher.mapply, line 88, in mapply
     * Module ZPublisher.Publish, line 42, in call_object
     * Module Products.WellGroup.WellGroup, line 110, in index_html
     * Module App.special_dtml, line 65, in __call__
     * Module DocumentTemplate.DT_String, line 476, in __call__
     * Module TreeDisplay.TreeTag, line 88, in render
     * Module TreeDisplay.TreeTag, line 192, in tpRender

KeyError: 'URL'

If I add a class variable called URL I then get a key error for RESPONSE.

My understanding from documentation is that Item (from which SimpleItem 
inherits) gives the product basic dtml-tree support. I can't find any 
examples that spell out how to write a product that supports dtml-tree 
tags. Could someone point me in the right direction?

Thanks,
Rowan