The current version is a corrected and somewhat revised version of the version published by Springer Verlag in 1988, which is out of print. This corrected version is made available with the permission of Springer-Verlag. The book has been converted from troff to LaTeX and many simplifications of notation have been made. Doubtless many conversion errors remain and I welcome any email pointing out definite or possible errors.
Permission is hereby given to freely print and circulate copies of this book so long as it is left intact and not reproduced for commercial purposes. The author would welcome all typos and comments.