private VarSymbol getDefinedSerializableFields(ClassSymbol def) {
        Name.Table names = def.name.table;

        /* SERIALIZABLE_FIELDS can be private,
         * so must lookup by ClassSymbol, not by ClassDocImpl.
         */
        for (Scope.Entry e = def.members().lookup(names.fromString(SERIALIZABLE_FIELDS)); e.scope != null; e = e.next()) {
            if (e.sym.kind == Kinds.VAR) {
                VarSymbol f = (VarSymbol)e.sym;
                if ((f.flags() & Flags.STATIC) != 0 &&
                    (f.flags() & Flags.PRIVATE) != 0) {
                    return f;
                }
            }
        }
        return null;
    }