There are several acknowledgements which should be made. Any mathematician
working in this area is conscious of his debt to the inventiveness of Thom, and to
the technical work of Mather which has placed much that was conjecture on a firm
mathematical foundation. As far as the seminar is concerned I would like to
single out the special contribution of Eduard Looijenga, who showed us how to fill
in gaps which otherwise might have remained open. Also, I would like to
acknowledge the considerable help offered by Terry Wall, from the inception of the
seminar to the production of the typescript. Further acknowledgements are due
to the seminar audience (who frequently had good reason to appear confused) for
their patience and in particular to Tim Ward who helped out with the talks; to
the British Scientific Research Council who provided financial support for
Eduard Looijenga, and the University of Wales whose financial assistance enabled
Andrew du Plessis to participate in the seminar; to Les Lander who was inveigled
into drawing the diagrams, and did an excellent job; to Dirk Siersma and
Klaus Lamotke whose careful reading of parts of the manuscript removed many errors;
and finally to Evelyn Quayle, Jean Owen and Margaret Walker who produced a
first-class typescript.