static <A, B> L<B> nValuesFromMultiMapByKeyDesc(int n, MultiMap<A, B> mm) {
  new L<B> out;
  for (A key : keysDesc(mm)) {
    int need = n-l(out);
    out.addAll(takeFirst(need, mm.get(key)));
    if (l(out) >= n) break;
  }
  ret out;
}