... implement a genetic programming system, or agents with evolvable code?

Genetic programming can be done in breve using the Push programming language. See the classes Push and PushGP for more information.

The demos PushTest.tz and PushRegression.tz show examples of using the Push programming language in breve.

The PushGP class is a pre-built GP engine which does not rely on the GeneticAlgorithm class. If you need more than a straightforward GP system, such as an environment in which agents use evolved code to interact with a simulation, you'll probably want to refer to the GeneticAlgorithm class and the FAQ entry on genetic algorithms for more information.