code: def _add_sub_tasks_to_executor(self, parent_task, parent_task_result):
          
          for sub_task in self.waiting_task_list.get_next_tasks(parent_task.id):
              self.executor.add_task(sub_task, parent_task_result)
docs: Add all subtasks for parent_task to the executor.
          :param parent_task: Task: task that has just finished
          :param parent_task_result: object: result of task that is finished