static <A> A resetFields(A object, S fields) {
  if (object != null) {
    O emptyInstance = newInstance(object.getClass());
    resetFields(object, fields, emptyInstance);
  }
  ret object;
}

static <A> A resetFields(A object, S fields, O emptyInstance) {
  for (S field : identifiers(fields))
    setOpt(object, field, getOpt(emptyInstance, field));
  ret object;
}