Subclassing is supported by the activity library as an integral technique for extending the framework it implements. There are two specific places where subclassing is expected as the normal technique of extension: definition of concurrent action groups, and definition of customized swarm objects. Concurrent action groups are not yet fully documented because their full interface is still being finalized. Swarm subclasses should inherit from the Swarm superclass defined in the objectbase library; the activity library provides the underlying support packaged by this superclass.