@Override
    public void visitMembers(ObjectVisitor visitor) {
        super.visitMembers(visitor);
        visitor.visit("id", id);
        visitor.visit("rank", rank);
        visitor.visit("aggregationresults", aggregationResults);
        visitor.visit("orderby-idx", orderByIdx);
        visitor.visit("orderby-exp", orderByExp);
        visitor.visit("children", children);
        visitor.visit("tag", tag);
    }
Adds an order-by expression to this group. If the expression is an AggregationResult, it will be added to the
list of this group's AggregationResults, and a reference to that expression is added instead. If the
AggregationResult is already present, a reference to THAT result is created instead.

@param exp the result to add
@param asc true to sort ascending, false to sort descending
@return this, to allow chaining
searchlib/src/main/java/com/yahoo/searchlib/aggregation/Group.java