static void ccopyFields(Concept x, Concept y, S... fields) {
  if (empty(fields)) { // assume we should copy all fields
    for (S field : conceptFields(x))
      cset(y, field, x.cget(field));
  } else 
    for (S field : fields) {
      O o = cset(x, field);
      if (o != null)
        cset(y, field, o);
    }
}