@Override
    public AggregationResult clone() {
        AggregationResult obj = (AggregationResult)super.clone();
        if (expression != null) {
            obj.expression = expression.clone();
        }
        return obj;
    }
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