private Set<String> getFilledSingle(String summaryName, Set<String> summaryNames, Iterator<Hit> iterator) {
        while (true) {
            if (summaryNames == null) {
                return Collections.singleton(summaryName);
            } else if (!summaryNames.contains(summaryName)) {
                return Collections.emptySet();
            }
            summaryNames = getSummaryNamesNextFilledHit(iterator);
        }
    }
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