[Zope] Newbie still having problems understanding namespaces

[David Lambert]

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