@Override
    protected void onSerialize(Serializer buf) {
        super.onSerialize(buf);
        serializeOptional(buf, expression);
        buf.putInt(null, tag);
    }
Mmust be implemented by subclasses to support merge. It is called as the {@link
#merge(AggregationResult)} method is invoked.

@param result the result to merge with
searchlib/src/main/java/com/yahoo/searchlib/aggregation/AggregationResult.java