static L<S> conceptFieldsInOrder(Concept c) {
  ret conceptFieldsInOrder(c.getClass());
}

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