妙法蓮華經玄義 6.209

6.209

若舊云：聖人以兩片無漏，夾熏一片有漏，練成無漏。今言：九次第定，熏修有漏成無漏，是那含業。四空定，是無色界業。

According to the old explanation: The noble ones use two sheets of the uncontaminated to sandwich and perfuse one sheet of the contaminated, refining it into the uncontaminated. We now say: The nine successive concentrations perfuse and cultivate the contaminated to make it uncontaminated; this is the karma of the anāgāmin. The four formless concentrations are the karma of the formless realm.