static Map<Class, LS> getFieldOrder_cache = weakMap();

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

static LS getFieldOrder(Class c) {
  if (c == null) null;
  ret getOrCreate(getFieldOrder_cache, c,
    () -> splitAtSpace(toStringOpt(getOpt(c, '_fieldOrder))));
}