DBSP as a language needs to compute over values in a group, which means that there must exist a minus operation and the ability to represent "negative" values, i.e., changes which delete. Z-sets can do this naturally. All primitive operators in the DBSP library compute over Z-sets and can handle both negative and positive changes (or a mix, in a Z-set some rows can be positive, some can be negative).
Unfortunately this requires a almost a complete redesign of the query engine; if it was any easier, we probably would have reused an existing engine.