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It is a pleasure to acknowledge the people who supported me during this work. First of all a special thanks is for Domenico Ursino who has never been just a supervisor but a great friend for me. My full gratitude goes to Luigi Palopoli who, with Domenico, has been a careful advisor and co-author of many papers; he supported me not only throughout the fulfillment of this work but also during some difficult moments of my life.

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Giorgio Terracina is Associate Professor in Computer Science at Universita´ delle Calabria. His research activities cover several areas from Information Systems, Artificial Intelligence and Bioinformatics. Results of such activities have been published in more than 100 papers on international journals and conferences.

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