
Acknowledgments
A. C. Κ. would like to express appreciation to his colleague O. Robert
Mitchell and his graduate students David Nahamoo, Carl Crawford, and
Kai-Wah Chan for many stimulating discussions. A number of our col-
leagues helped us by providing figures (see the individual figure credits).
In addition, many of the figures were produced by Andrew Pilipchuk
and others at the University of Maryland; and by Carl Crawford, Doug
Morton, and Kai-Wah Chan at Purdue University. Kathryn Riley, Dawn
Shifflett, and Mellanie Boes, among others, did an excellent job of pre-
paring the text. To these, and to others too numerous to mention, our
sinceres ...