The authors extend their thanks to Sigrun Torrissen, Gabriella Nagy, Sigmund K. Runestad, Finn Moe, and Linn M. Johansen for their careful and accurate technical assistance and to Rune Goa and Per Torgersen for preparation of the illustrations. They also thank Paul Grogan, Eystein Jansen, Anders Solheim, and Robert Williams for useful discussions and suggestions.