BooksecrCollected Volumepp. xi–xivIntroductionAntonio CampilloUniversidad de Valladolid, SpainzbMATHMRFranz-Viktor KuhlmannUniversity of Saskatchewan, Saskatoon, CanadazbMATHBernard TeissierInstitut Mathématique de Jussieu-Paris Rive Gauche, FrancezbMATHDownload Chapter PDF