results which I used for developing the classification of this paper  Her work has been very relevant in reaching my results I would also like to thank Bernhard Nebel for some important observations he made to a student of mine  Alessandro Fin  which improved this work  Further thanks go to Jochen Renz for discussions in the early stages of this work I would also like to thank the anonymous referees of the Journal of Artificial Intelligence Research for their very careful comments and suggestions which allowed me to make the presentation simpler and more systematical Some of the proofs in the paper have been rewritten thanks to their suggestions Finally I would like to thank Tony Cohn for reading a near final version  His comments have been useful in enhancing both scientific and literary quality of the paper This work has been developed in the context of the National Project  MURST ex Metodologie e tecnologie per la gestione di dati e processi su reti internet ed intranet" Methods and technologies for data and process management on internet and intranet directed by L  Tanca