We wish to thank A.S. Stepanenko for useful discussions. This work was partially supported (K.N.I.) by Grant R4TOOO of the International Science Foundation;
Grants 94-02-03712 and 95-01-00548 of the Russian Fund of Fundamental Investigations;
an Euler stipend of the German Mathematical Society;
INTAS-939 and by the UK EPSRC Grants GR/J35221 and GR/K68356;