[Zope] Keyword index negative query

Hi, I'm using Zope-2.6.1 with ZCatalog and several Keyword indexes. How can I make a query on a KeywordIndex to get objects for which a given value is not found ?? Typically, each of my objects is identified by a globally unique ID (OID) ; an object can keep a reference to another object through an "oid_refs" property (a list), which is indexed in a keyword index. What I want is to be able to find, for example, objects of a given meta-type, eventually stored in a given folder, which are NOT referenced by other objects. Thanks for any help, Thierry -- Linux every day, keeps Dr Watson away... http://gpc.sourceforge.net -- http://www.ulthar.net