code: def connect(self, task_spec):
          
          assert self.default_task_spec is None
          self.outputs.append(task_spec)
          self.default_task_spec = task_spec.name
          task_spec._connect_notify(self)
docs: Connects the task spec that is executed if no other condition
          matches.
  
          :type  task_spec: TaskSpec
          :param task_spec: The following task spec.