[Zope] Force DTML evaluation?

I created a Renderable derived ZClass to act as a "content box". I am basically providing title and content attributes, and when called like <dtml-var myBox> it nicely renders into an HTML table. Then I realised I could not use DTML in the content (type: text). duh! Is there a function I could use like like <dtml-var "do_the_dtml_magic_with(content)">? My second try was to derive from DTMLDocument but I could for the heck of it not figure out how to call the baseclass' methods. In python I would use baseclassname.method() but this seems impossible in ZClasses... Any insights appreciated, Stefan