[Zope] Expressing expressions

[RC Compaan]

Simple addition seems a bit tricky in zope

  <dtml-call "REQUEST.set('CategoryTotal',0)">

  <dtml-in "tmp.propertyItems()">
     <dtml-let key="_['sequence-key']"
               value="_['sequence-item']"
               category="s[:1]"
               counter="_.getitem(category+'Total')">  <--// Here i want to
concatenate the category from the value with the word total so that i can
lookup my declared variable in the namespace.
         <dtml-var expr="counter=counter+value">  <--// Here i tried to
increase the declared variable with the value.
     </dtml-let>
  </dtml-in>

Can someone help?

Roché