public static boolean isSimpleFeature(Reference reference) {
        if ( ! reference.isSimple()) return false;
        if (reference.output() != null) return false;
        String name = reference.name();
        return name.equals("attribute") || name.equals("constant") || name.equals("query");
    }
Returns true if the given reference is an attribute, constant or query feature
searchlib/src/main/java/com/yahoo/searchlib/ranking/features/FeatureNames.java