Bosun how to reserve all groups when calculate two numberSet

Question

There are two numberSet numberSetA: { "group1": 3 }

numberSetB: { "group2": 5 }

how can i get the result like { "group1": 3, "group2": 5 } when numberSet joining groups, use 0 if the group does not exist in this numberSet

Basic Operators will generate a "unjoined group" when group does not exist in any numberSet, what can i do