Acknowledgments. We are deeply indebted to Professor Maurizio Barbieri for revising the English form;
to Dr. Fabio Triolo for critical reading of the manuscript;
and to Mr. Salvatore Buccoleri for his assistance in preparing drawings and ®gures. This work was partially supported by funds of the Italian Ministero dell'UniversitaÁ e della Ricerca Scienti®ca (MURST 60 and 40%);