Note: A zipped up postscript version of this paper is also available.
Pieter J. Mosterman 
 
Institute of Robotics and System Dynamics 
 
 German Aerospace Establishment 
 
 D-82230 Wessling 
 
 Germany 
 
 Pieter.J.Mosterman@dlr.de 
 
 - 
Gautam Biswas 
 
 Center for Intelligent Systems 
 
 Vanderbilt University 
 
 Nashville, TN  37235
 
U.S.A. 
 
biswas@vuse.vanderbilt.edu