ABSTRACT
This paper is a development of a draft version that arose from a longstanding exchange of ideas bringing together four people: Madeleine Baranger, Alberto Campo, Jorge Mom, and myself. The other three persons whom I have just mentioned could well have been the co-authors of this text; they allowed me, however, to add my own developments to our initial draft. As a result, certain conclusions that I have drawn may not be shared by all four of us. Their contribution not only to the overall discussion but also to many points of detail is nonetheless a significant one, and I am grateful to them for it.
