This work was partially supported by a grant from Simons Foundation
which allowed me a stimulating working environment during
my visit at the Isaac Newton Institute
of Cambridge, UK (EPSRC grant EP/K032208/1).
It is a pleasure to thank Dino Esposito for the
long discussions on several of the important points touched upon here and that
we first wrote in our dialogue book (16), and for his patient
and accurate review of the manuscript, which has also benefitted of
comments by Norman Fenton and Geert Verdoolaege.
