static HashSet collectUnique(Collection c, S field) {
  new HashSet set;
  for (O a : c) {
    O val = getOpt(a, field);
    if (val != null)
      set.add(val);
  }
  ret set;
}

static HashSet mapMethodLike collectUnique(S field, Collection c) {
  ret collectUnique(c, field);
}