《元史·列传·卷六十二》
 君父之仇，不共戴天，其所以如此，是要明纲常伦理，别上下之分。
君父之仇，不共戴天，所以明纲常、别上下也。
The hatred between the monarch and his father is irreconcilable. The reason for doing this is to clarify the principles of morality and distinguish between superiors and inferiors.