《后汉书·列传·张曹郑列传》
按礼记所载，为人后者就算是他的儿子，既然侍奉元帝为大宗，就应降其私亲。
礼，为人后者则为之子，既事大宗，则降其私亲。
As stated in the Rites, a person's descendants are considered to be his children, so since the emperor served as the grand ancestor to Emperor Yuan, his own close relatives should be lowered in rank.