1. Dept. of Mathematics, Univ. of Oslo, Mech. Div., P.O. Box 1053, Blindern, N-0316 Oslo, Norway; formerly, Res. Asst., School of Mathematics, Univ. of Bristol, University Walk, Bristol BS8 1TW, U.K. E-mail: deborah@math.uio.no
2. Prof. of Appl. Mathematics, School of Mathematics, Univ. of Bristol, University Walk, Bristol BS8 1TW, U.K. E-mail: D.H.Peregrine@ bristol.ac.uk
3. Lect., School of Mech. Engrg., Univ. of Edinburgh, Sanderson Build., King's Build., Edinburgh EH9 3JL, U.K. E-mail: Tom.Bruce@ed.ac.uk