1. University Lecturer, Engineering Design Centre, Cambridge Univ. Engineering Dept., Trumpington St., Cambridge, CB2 1PZ, UK. E-mail: ks273@cam.ac.uk
2. Professor, Applied Computing and Mechanics Laboratory, Station 18, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne EPFL, Switzerland. E-mail: ian.smith@epfl.ch