Due to the nature of More or Less and Grusilda, I don't expect the new interactions between those two cards, including the "combining" mechanic in Grusilda, to be covered to any extent in the comprehensive rules, let alone rigorously. Unlike with bona fide Magic cards, thisThe interaction between Grusilda and More or Less, as with other situations unique to Unstable cards, is a gap that is rather deliberately meant to be filled in by the players in a particular game — usually in the spirit of having fun.
EDIT (Jan. 16): Correctness edit in view of changes to the Magic Tournament Rules with Rivals of Ixalan.
Due to the nature of More or Less and Grusilda, I don't expect the new interactions between those two cards, including the "combining" mechanic in Grusilda, to be covered to any extent in the comprehensive rules, let alone rigorously. Unlike with bona fide Magic cards, thisThe interaction between Grusilda and More or Less, as with other situations unique to Unstable cards, is a gap that is rather deliberately meant to be filled in by the players in a particular game — usually in the spirit of having fun.EDIT (Jan. 16): Correctness edit in view of changes to the Magic Tournament Rules with Rivals of Ixalan.