@Override
protected void retrieveGroupHeadAndAddIfNotExist(int doc) throws IOException {
filler.fillValue(doc);
GroupHead groupHead = groups.get(mval);
if (groupHead == null) {
MutableValue groupValue = mval.duplicate();
groupHead = new GroupHead(groupValue, sortWithinGroup, doc);
groups.put(groupValue, groupHead);
temporalResult.stop = true;
} else {
temporalResult.stop = false;