This work was written during my visits at CORE (Belgium), at the Faculty of Economics and Politics in Cambridge (England), and at the Department of Mathematics at the ETH in ZUrich. I wish to thank J.H. Dr~ze (CORE) for most help­ ful suggestions, and I am indebted to H. BUhlmann (ETH) for his advice and for encouragement. The comments by M. Granzio1, M. Janssen and by anonymous referees were very useful. However, I assume the responsibility for remaining errors. I am grateful to R. Boller, I. Lather and M. Urfer for their careful typing of the manuscript. Support by the SWiss National Science Foundation is acknowledged.