[Zope] How to assign a value to a dtml variable after it has been create d with the let dtml tag ?

[Bocquillon Philippe]

Hello,

In the piece of DTML code here after, I'm trying to compare each element of
getAllPortalCategories() with each of getCategories().
I would like to have each element as a selected option in the generated HTML
if this element is in both lists.
Here the error I get is that Zope searches for a variable named 'false'. 
I would want Zope considers 'false' as a string value assigned to
isSelected, not as a var to search.
But my problem is more general: how can I modify the value of a dtml
variable? 

<tr>
      <th align="LEFT" valign="TOP">Categories (current ones are
selected)</th>
</tr>
<td align="LEFT" valign="TOP">
    <select multiple name="categories:list" size=10>
    <dtml-in expr="getAllPortalCategories()">
        <dtml-let aPortalCategory=sequence-item isSelected='false'> 
            <dtml-in expr="getCategories()">
                <dtml-if expr="aPortalCategory == _['sequence-item']">
                    _['isSelected']='true'
                </dtml-if>
            </dtml-in>  
            <dtml-if expr="_[isSelected]=='true'">
                <option selected> <dtml-var aPortalCategory> </option>
            <dtml-else>
                <option> <dtml-var aPortalCategory> </option>            
            </dtml-if>
        </dtml-let>
    </dtml-in>  
</td>

Thanks in advance,

Philippe