Department of Computing, University of the West of England, Bristol, U.K. and Interface Technology Research Limited, CRII DuPont Centre, Bristol, U.K.;Human Factors Dept., Sowerby Research Centre, British Aerospace (Operations Ltd.) Bristol, U.K., Interface Technology Research Limited, CRII DuPont Centre, Bristol, U.K.;Human Factors Dept., Sowerby Research Centre, British Aerospace (Operations Ltd.) Bristol, U.K. and Interface Technology Research Limited, CRII DuPont Centre, Bristol, U.K.;Department of Computing, University of the West of England, Bristol, U.K. and Interface Technology Research Limited, CRII DuPont Centre, Bristol, U.K.;Department of Computing, University of the West of England, Bristol, U.K. and Interface Technology Research Limited, CRII DuPont Centre, Bristol, U.K.