def _add_sub_tasks_to_executor(self, parent_task, parent_task_result):
        """
        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
        """
        for sub_task in self.waiting_task_list.get_next_tasks(parent_task.id):
            self.executor.add_task(sub_task, parent_task_result)
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