Acknowledgements

This research is sponsored by the Austrian Fonds zur Förderung der Wissenschaftlichen Forschung (FWF) under grant number J01443-INF (Schrödinger-Stipendium). I would like to thank Hendrik Blockeel, William Cohen, Tom Mitchell, Bernhard Pfahringer, Michèle Sebag, Ashwin Srinivasan, Gerhard Widmer, and the anonymous reviewers of this and previous versions of the paper for helpful suggestions, discussions and pointers to relevant literature. Thanks are also due to Ray Mooney for making his Common LISP Machine Learning library available and to the maintainers of the UCI Machine Learning repository.