B. J. Coomer* and R. Jones
Department of Physics, The University of Exeter, Exeter, EX4 4QL, United Kingdom
Department of Mathematics, University of Luleå, Luleå, S95 187, Sweden
P. R. Briddon
Department of Physics, The University of Newcastle upon Tyne,
Newcastle upon Tyne, NE1 7RU, United Kingdom
EPSRC and the ENDEASD network are thanked for computer
time and financial support respectively.
** Financially support by NFR and TFR, and computer support by PDC at KTH in Sweden are gratefully acknowledged.