[Zope] Concatenation

[Ben Ocean]

Hi;
I'm trying to concatenate a <dtml-var> with a string. I have this poor attempt:
   <dtml-var expr="_.string(title_or_id.'_frame')(_,_.None)">
where *title_or_id* is (of course) the <dtml-var> and *_frame* is the 
string. How should I do this?
TIA,
BenO