static <A, B> Set<A> allKeys(Iterable<Map<A, B>> l) {
  Set<A> set = null;
  fOr (Map<A, B> map : l) {
    if (map == null) continue;
    if (set == null) set = similarEmptySet(map);
    addAll(set, keys(map));
  }
  ret set;
}