@Override
    public int compare(Hit left, Hit right) {
        if (isHitGroup(left) ^ isHitGroup(right)) {
            return isHitGroup(left) ? 1 : -1;
        } else {
            return super.compare(left, right);
        }
    }
Ensures that HitGroups are placed last in the result.

@author Tony Vaagenes
container-search/src/main/java/com/yahoo/search/result/HitGroupsLastComparator.java