Authors are grateful to D.Barkley for fruitful and encouraging discussions, and to B.Fiedler, M.Golubitsky and C.Wulff for letting us know about their work prior to publication. This work was supported in part by The Wellcome Trust Collaborative Biomedical Research Fellowship (045192) for VNB, EPSRC-AMS (GR/K49775) and Russian Fund for Basic Research (96-01-00592).