static Map<Class, LS> fieldsInOrder_cache = weakHashMap();

static LS fieldsInOrder(O o) {
  ret fieldsInOrder(_getClass(o));
}

static LS fieldsInOrder(Class c) {
  if (c == null) null;
  synchronized(fieldsInOrder_cache) {
    LS fields = fieldsInOrder_cache.get(c);
    if (fields != null) ret fields;
    S order = toStringOpt(getOpt(c, "_fieldOrder"));
    Set<S> set = asTreeSet(allNonStaticFields(c));
    if (order == null) ret asList(set);
    fields = splitAtSpace(order);
    setAddAll(fields, set); // TODO: optimize
    fieldsInOrder_cache.put(c, fields);
    ret fields;
  }
}

Began life as a copy of #1019416