In this example, the agent does only two things: sense the world and then flees if it senses a predator. Only one behavior is possible: flee. flee is released by the presence of a predator. A predator is of type Releaser and has only two possible values: it is either present or it is not. If the agent does not sense the releaser for the behavior, the agent does nothing. There is no “default” behavior.
This example also shows filtering of perception. In the above example, the agent only looks for predators with a dedicated detection function, sense- Predators().