The authors would like to express their gratitude to the anonymous reviewers of the paper. Their contributions toward improving the quality of this paper are relevant enough to be considered, in some sense, as a co-authors of the paper. The shortcomings still in the paper can only be attributed to the nominal authors.

The second author has been partially supported by the Spanish Ministerio de Ciencia y Tecnología and FEDER funds, under the project DPI2003-05193-C02-01 of the Plan Nacional de I+D+I.

Josep M Porta 2005-02-17