Returns the value JUST BEFORE the merge.
Returns the value JUST BEFORE the merge. If it is empty, it is like a zero. the key should hold: val (k,v) = kv result = get(k) key is set to: result.map(Semigroup.plus(_, Some(v)).getOrElse(v) after this.
merge a set of keys.
Main trait to represent stores that are used for aggregation