[Zope] Find all *empty* values using ZCatalog?

[Itamar Shtull-Trauring]

I would like to search using ZCatalog for all objects that have an empty
attribute, e.g.
<dtml-in "catalog({'history': ''})">

only this returns all objects in catalog.

Any ideas?
I do not want to do

<dtml-in catalog>
<dtml-unless history>
...
</dtml-unless>
</dtml-in>

-- 
Itamar S.T.  itamars@ibm.net