code: def add(self, task):
          
          wait_id = task.wait_for_task_id
          task_list = self.wait_id_to_task.get(wait_id, [])
          task_list.append(task)
          self.wait_id_to_task[wait_id] = task_list
docs: Add this task to the lookup based on it's wait_for_task_id property.
          :param task: Task: task to add to the list