It is a pleasure to acknowledge José M. Ibáñez, Ewald Müller and
Philippos Papadopoulos for valuable suggestions and a careful
reading of the manuscript. I also want to thank Tom Goodale for a
meticulous revision of the English of the final draft. I am
particularly grateful to Konstantinos Kifonidis, José V. Romero
and Wai-Mo Suen for providing some of the movies contained in
this paper. This work has been supported in part by a TMR
fellowship from the European Union (contract nr.
ERBFMBICT971902). The author has made use of NASA's Astrophysics
Data System Abstract Service which is gratefully acknowledged.