Although it's not implimented you could create a Counter CRDT using an array of numbers, appending each addition or negation, the 'value' being the sum of the a

Although it's not implimented you could create a Counter CRDT using an array of numbers, appending each addition or negation, the 'value' being the sum of the array. This can be compressed by each client changing its most recent value if a sync has not been sent. There is probably also other optimisations you can do too, probably around keeping track of which client made which change and updating its value.