This second edition of my thesis was created in 1993 and appeared in
1996 as CWI tract 109.
It contains the same material as the original Ph.D. thesis.
In addition a large amount of follow-up work is discussed and referenced.
Copies can be obtained through your bookstore,
or <A HREF=http://www.cwi.nl/static/publications_bibl/tract_index.html>from CWI</A>.