// o is either a map already (string->object) or an arbitrary object,
// in which case its fields are converted into a map.
static Map<S, O> objectToMap_inOrder_withoutFields(O o, Collection<S> withoutFields) {
  if (o == null) null;
  if (o instanceof Map) ret (Map) o;
  new LinkedHashMap<S, O> map;
  for (S field : listWithoutSet(fieldsInOrder(o), withoutFields))
    map.put(field, get(o, field));
  ret map;
}