  Qu'est la majest{\'e} de ce qui finit, aupr{\`e}s des d{\'e}parts titubants,
des d{\'e}sordres de l'aurore?. Let this question from the French writer
Colette guide us to walk through the promising beginnings, fascinating
evolutions and results, challenging open problems and perspectives of an
exciting theory that is far from revealing all its secrets.
