public void setFilled(String summaryClass) {
        if (filled == null || filled.isEmpty()) {
            //TODO Should have used Set.of(), but it checks that contains is not called with null
            filled = Collections.singleton(summaryClass);
            unmodifiableFilled = filled;
        } else if (filled.size() == 1) {
            filled = new HashSet<>(filled);
            unmodifiableFilled = Collections.unmodifiableSet(filled);

            filled.add(summaryClass);
        } else {
            filled.add(summaryClass);
        }
    }
Register that this hit has been filled with properties using
the given summary class. Note that this method will implicitly
tag this hit as fillable if it is currently not.

@param summaryClass summary class used for filling
container-search/src/main/java/com/yahoo/search/result/Hit.java