> 2. A single tree which returns multiple values
> For the second case, you'd need GPNodes which return not one but two numbers -- just modify the GPData to have two instance variables as appropriate. This may or may not make sense for your problem.
Just a suggestion -- this could be a job for Cartesian GP or similar
graph-based GP. The ECJ homepage has a link to an ECJ implementation
of Cartesian GP.
Dr. James McDermott
Natural Computing Research & Applications Group,
Complex and Adaptive Systems Laboratory,
University College Dublin, Ireland.