sO safeUnstructureAllowingClasses(S s, Class... allowedClasses) {
  final Set<S> allowedClassesSet = mapToSet className(allowedClasses);
  O classFinder = func(S name) {
    if (!allowedClassesSet.contains(name)) fail("Class not allowed to unstructure: " + name);
    ret classForName(name);
  };
  ret unstructure(s, false, classFinder);
}