ACKNOWLEDGEMENTS

Thanks are due to Dr. M.T. Lamata (Granada) for discussing the mathematical basis of this paper. Dr. P. Mein (Lyon) gave us access to his collections. Dr. M. Hugueney (Lyon) provided us with several unpublished profile drawings. This study was executed in the framework of research group RNM 0190 of the 'Junta de Andalucia' and the project Consolider Ingenio 2010, CSD 2006-00041. We wish to thank two anonymous referees who suggested many improvements of this paper.