Published online by Cambridge University Press: 23 November 2009
* I am indebted to Professors Richard Hunter and Stephanos Efthymiadis for their helpful comments. Special gratitude is due to Professor Marco Fantuzzi, without whose constant encouragement this paper would never have been written. Last but not least, the suggestions of CQ's anonymous referee, and especially his/her doubts, are noted with thanks in the course of the discussion.