presto-main/src/main/java/io/prestosql/type/setdigest/MergeSetDigestAggregation.java/MergeSetDigestAggregation/merge
class MergeSetDigestAggregation:    private static void merge(SetDigestState state, SetDigest instance)
    {
        if (state.getDigest() == null) {
            state.setDigest(instance);
        }
        else {
            state.getDigest().mergeWith(instance);
        }
    }