This is the theory arguing that Fukai-no-Joten/ Fukaijoten is a code to rule imperial succession, however, it does not decide who should take the Throne but establishes the practice in which the former emperor appoints and chooses his successor.
不改常典は皇位継承についての規定だが、誰がつくかを指定したものではなく、前天皇が後継者を指名して選ぶことを定めたものだとする説である。