private Set<String> getSummaryNamesNextFilledHit(Iterator<Hit> hitIterator) {
        while (hitIterator.hasNext()) {
            Set<String> filled = hitIterator.next().getFilled();
            if (filled != null)
                return filled;
        }
        return null;
    }
Returns the set of summaries for which all concrete hits recursively below this is filled.
container-search/src/main/java/com/yahoo/search/result/HitGroup.java