School of Mathematical and Computer Sciences, Heriot-Watt University Edinburgh EH14 4AS, UK. ceekk@macs.hw.ac.uk|School of Mathematical and Computer Sciences, Heriot-Watt University Edinburgh EH14 4AS, UK. ceekk@macs.hw.ac.uk|School of Mathematical and Computer Sciences, Heriot-Watt University Edinburgh EH14 4AS, UK. ceekk@macs.hw.ac.uk