@Override
        public void execute(Object obj) {
            AggregationRefNode ref = (AggregationRefNode)obj;
            int idx = ref.getIndex();
            if (idx < 0) {
                AggregationResult res = ref.getExpression();
                idx = indexOf(res);
                if (idx < 0) {
                    idx = results.size();
                    results = add(results, res);
                }
                ref.setIndex(idx);
            } else {
                ref.setExpression(results.get(idx));
            }
        }
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