[Zope] Sum up a property from several objects

[Gitte Wange]

Hello,

I have a folder contaning some objects.
These objects all have a property named CPU (it's an int).

Now I want to iterate through the folder's objects and sum up the value of
this CPU property.

I have tried with this - IMHO - really bad code:

<dtml-with my_computers>
 <dtml-in "objectValues()">
  <dtml-if sequence-start>
   <dtml-call "REQUEST.set('new_cpu',CPU)">
  <dtml-else>
   <dtml-let cpu="new_cpu + CPU">
    <dtml-call "REQUEST.set('new_cpu',cpu)">
   </dtml-let>
  </dtml-if>
 </dtml-in>
</dtml-with>

But I get this error:
Error Type: KeyError
Error Value: cpu

Any other way to do this ?
(I am pretty sure there is - it's just me missing something basic here)

Regards,
Gitte Wange