public void setFillable() {
        if (filled == null) {
            //TODO Should have used Set.of(), but it checks that contains is not called with null
            filled = Collections.emptySet();
            unmodifiableFilled = filled;
        }
    }
Tag this hit as fillable. This means that additional properties
for this hit may be obtained by fetching document
summaries. This also enables tracking of which summary classes
have been used for filling so far. Invoking this method
multiple times is allowed and will have no addition
effect. Note that a fillable hit may not be made unfillable.
container-search/src/main/java/com/yahoo/search/result/Hit.java