Hendrik Blockeel is a post-doctoral fellow of the Fund for Scientific Research (FWO) of Flanders. Jan Ramon is funded by the Flemish Institute for the Promotion of Scientific Research in Industry (IWT). Henk Vandecasteele was funded in part by the FWO project G.0246.99, ``Query languages for database mining''. The authors thank Luc De Raedt for his influence on this work, Ashwin Srinivasan for suggesting the term ``query packs'', the anonymous reviewers for their useful comments, and Kurt Driessens for proofreading this text. This work was motivated in part by the Esprit project 28623, Aladin.