meta-for MultiMap also as MultiSetMap {
  static <A, B> MultiMap<A, B> keysWithMultipleValuesOnly(MultiMap<A, B> mm) {
    new MultiMap<A, B> mm2;
    for (A key : mm.keySet())
      if (l(mm.get(key)) > 1)
        mm2.putAll(key, mm.get(key));
    ret mm2;
  }
}