// the problem is that they provide only a Map not a Set
// a direct replacement of LinkedHashSet would require a wrapper
static <A, B> MultiSetMap<A, B> multiSetMap_outerDescTreeMap_innerApacheLinkedHashSet() {
  ret new MultiSetMap<A, B>(descTreeMap()) {
    Set<B> _makeEmptySet() { ret apacheLinkedHash; }
  };
}