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.