public Set<String> getFilled() {
        return unmodifiableFilled;
    }
Returns the set of summary classes for which this hit is
filled as an unmodifiable set. If this set is 'null', it means that this hit is
unfillable, which is equivalent to a hit where all summary
classes have already been used for filling, or a hit where
further filling will yield no extra information, if you prefer
to look at it that way.

Note that you might need to overload isFilled if you overload this one.
container-search/src/main/java/com/yahoo/search/result/Hit.java