[Zope] Newbie still having problems understanding namespaces

I'm sure this is a case of RTFM, but I think I may be reading the wrong FM! Looking at the DTML reference section n the Zope Book (PDF version) on page 228, I see a set of DTML functions which I had assumed I could call, for example: <dtml-var expr="abs(-23)"> which I would have expected to evaluate to 23. Instead I get the error: Error Type: NameError Error Value: global name 'abs' is not defined. I guess this is because abs is not in the current nemespace. My question is where is the documentation that tells me which namespace to use and when? TIA Dave Lambert