static TreeSet<S> collectInCISet(Iterable c, S field) {
  TreeSet<S> set = ciSet();
  for (O a : c) {
    O val = getOpt(a, field);
    if (val cast S)
      set.add(val);
  }
  ret set;
}

static TreeSet<S> collectInCISet(S field, Iterable c) {
  ret collectInCISet(c, field);
}