@Override
    protected void onDeserialize(Deserializer buf) {
        super.onDeserialize(buf);
        expression = (ExpressionNode)deserializeOptional(buf);
        tag = buf.getInt(null);
    }
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