ACKNOWLEDGMENTS
We thank our reviewers and the Editor for comments on the manuscript. We also thank Hannah Bonner for linguistic correction of an earlier English version of the paper. This work is a contribution to projects CGL2009-7896/BTE and CGL2010-17889/BTE of Dirección General de Investigación, Ministerio de Ciencia e Innovación of Spain, and SGR324 (Generalitat de Catalunya). One of the authors (P.B.) receives support from a JAE-DOC (CSIC) contract (Junta para la Ampliación de Estudios).
Handling editor: Robert Asher