Plain TeX 12+1 pages with 3 tables in the text, 3 .eps figures.
  Presented by G.V. at "EURIDICE: The Final Meeting", Kazimierz, Poland, August
  24 -- 27, 2006; v2: minor (one word!) editing by referee; v3: final version
  to appear in Acta Physica Polonica B 38, n.9 (2007), with further minor
  corrections