private Set<String> getFieldsForClass(String wantClass) {
        if (! ensureKnownClass(wantClass)) {
            throw new IllegalArgumentException("unknown summary class: " + wantClass);
        }
        return knownSummaryClasses.get(wantClass);
    }
resolve summary class to use when none provided
container-search/src/main/java/com/yahoo/prelude/fastsearch/PartialSummaryHandler.java