int indexOf(AggregationResult lhs) {
            int prevTag = lhs.getTag();
            for (int i = 0, len = results.size(); i < len; ++i) {
                AggregationResult rhs = results.get(i);
                lhs.setTag(rhs.getTag());
                if (lhs.equals(rhs)) {
                    return i;
                }
            }
            lhs.setTag(prevTag);
            return -1;
        }
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