static L<S> fieldsInOrder(O o) {
  ret fieldsInOrder(_getClass(o));
}

static L<S> conceptFieldsInOrder(Class c) {
  if (c == null) null;
  S order = toStringOpt(getOpt(c, "_fieldOrder"));
  Set<S> set = asTreeSet(fields(c));
  if (order == null) ret asList(set);
  L<S> fields = splitAtSpace(order);
  setAddAll(fields, set);
  ret fields;
}