Decrements .task.todo; if it thus falls to <= 0, enacts method whose
name is 'task_' + self.task.name and sets .task = None. If is_AI, calls
.decide_task to decide a self.task.
Decrements .task.todo; if it thus falls to <= 0, enacts method whose
name is 'task_' + self.task.name and sets .task = None. If is_AI, calls
.decide_task to decide a self.task.