One way to make query answering system (QAS) intelligent is to assume a hierarchical structure of its attributes. Such systems have been investigated by Cuppens & Demolombe (1988), Gal & Minker (1988), and Gaasterland et al. (1992), and they are called cooperative. Any attribute value listed in a query, submitted to cooperative QAS, is seen as a node of the tree representing that attribute. If QAS retrieves an empty set of objects, which match query q in a target information system S, then any attribute value listed in q can be generalized and the same the number of objects that possibly can match q in S can increase. In cooperative systems, these generalizations are usually controlled by users.