public static Optional<String> argumentOf(String feature) {
        Optional<Reference> reference = Reference.simple(feature);
        if ( reference.isEmpty()) return Optional.empty();
        if ( ! ( reference.get().name().equals("attribute") ||
                 reference.get().name().equals("constant") ||
                 reference.get().name().equals("query")))
            return Optional.empty();

        return Optional.of(reference.get().arguments().expressions().get(0).toString());
    }
Returns the single argument of the given feature name, without any quotes,
or empty if it is not a valid query, attribute or constant feature name
searchlib/src/main/java/com/yahoo/searchlib/ranking/features/FeatureNames.java