static <A> A copyFields(O x, A y, S... fields) {
  if (empty(fields)) { // assume we should copy all fields
    Map<S, O> map = objectToMap(x);
    for (S field : map.keySet())
      setOpt(y, field, map.get(field));
  } else 
    for (S field : fields) {
      O o = getOpt(x, field);
      if (o != null)
        setOpt(y, field, o);
    }
  ret y;
}

static <A> A copyFields(O x, A y, Collection<S> fields) {
  ret copyFields(x, y, asStringArray(fields));
}