[Zope] ZCatalog and boolean values

Hi, I have an index on a boolean property named 'singlelang' in a ZCatalog. Searching for all objects having 'singlelang="on"' ('true' is called 'on' in Zope; I don't know why, but that's another story) is easy. But I also would like to be able to find all objects, where the property 'singlelang' is 'false' (or 'off'). But this doesn't seem to be possible. Is there any solution to this problem? Roland