the discussions could not, unfortunately, be included. We thank the staff of the castle, and especially its administrator, Mr. Hormann, for providing the excellent environment conducive to smooth run­ ning of the conference. We acknowledge the Max-Planck-Gesellschaft for its support in financing the workshop. Thanks are also due to all participants for their enthusiasm and (generally!) prompt submission of manuscripts. Last but not least, we thank Gabriele Breuer for her invaluable help before, dur­ ing and after the days at SchloB Ringberg. She skillfully typed and retyped many of the manuscripts for these proceedings and unstintingly performed last minute corrections. We hope that some of the ideas presented here will stimulate further discussion and research in this area. R.Beck Bonn, December 1986 R. Grave v 1 Arp, H. 15 Kundt, W. 29 Furst, E. 2 Grave, R. 16 Aly, J.J.