《资治通鉴·宋纪·宋纪六》
拓跋焘北征时，刘暗中对与他亲近的人说： 如果车驾回不来了，我就拥立乐平王拓跋丕做皇帝。 
帝之北行也，絜私谓所亲曰： 若车驾不返，吾当立乐平王。 
When Tuoba Dao went on the Northern Expedition, Liu secretly told someone close to him: If the Chariot cannot return, I will make King Lepin, Tuoba Pi, the emperor.
<s>[INST] Kindly elucidate the central themes in this passage in English: 帝之北行也，絜私谓所亲曰： 若车驾不返，吾当立乐平王。  [/INST] When Tuoba Dao went on the Northern Expedition, Liu secretly told someone close to him: If the Chariot cannot return, I will make King Lepin, Tuoba Pi, the emperor.</s>
Kindly elucidate the central themes in this passage in English: 帝之北行也，絜私谓所亲曰： 若车驾不返，吾当立乐平王。
user
When Tuoba Dao went on the Northern Expedition, Liu secretly told someone close to him: If the Chariot cannot return, I will make King Lepin, Tuoba Pi, the emperor.
assistant