The recursive case (it isn't really recursive in concept, just applying the rule consistently) is a lot more elegant, and there's a fairly simple use case: there's a statement Fact, then a statement Certify that certifies it is true (Fact is very high stake and requires careful verification), then a statement Void that voids the certification. If a subset-interested system synced Fact and Certify at one point (via a subsetting that would normally only include Fact without this rule), it needs to later sync Void (or be missing very important information).