  This is the typewritten version of a handwritten manuscript which was
completed by Ian G. Macdonald in 1987 or 1988. The manuscript is a very
informal working paper, never intended for formal publication. Nevertheless,
copies of the manuscript have circulated widely, giving rise to quite a few
citations in the subsequent 25 years. Therefore it seems justified to make the
manuscript available for the whole mathematical community. The author kindly
gave his permission that a typewritten version be posted on arXiv.
