// analogous to OptimizedMultiSet
static class MultiSetMapWithTopTen<A, B> extends MultiSetMap<A, B> {
  // outer tree map for sorting
  // inner linked hash set for reproducability (HashSet ordering sucks)
  MultiSetMap<Int, A> byCount = multiSetMap_innerLinkedHashSet_outerRevTreeMap();

  *() {}
  *(bool useTreeMap) { super(useTreeMap); }
  *(MultiSetMap<A, B> map) { super(map); }
  *(Map<A, Set<B>> data) { super(data); }
  
  private void unindex(A key, Set<B> values) {
    if (nempty(values)) byCount.remove(l(values), key);
  }

  private void index(A key, Set<B> values) {
    if (nempty(values)) byCount.add(l(values), key);
  }

  bool put(A key, B value) { synchronized(data) {
    Set<B> set = data.get(key);
    unindex(key, set);
    if (set == null)
      data.put(key, set = _makeEmptySet());
    bool changed = set.add(value);
    index(key, set);
    if (!changed) false;
    ret true with ++size;
  }}

  void remove(A key) { synchronized(data) {
    Set<B> set = data.get(key);
    if (set == null) ret;
    size -= l(set);
    unindex(key, set);
    data.remove(key);
  }}

  void remove(A key, B value) { synchronized(data) {
    Set<B> set = data.get(key);
    if (set == null) ret;
    unindex(key, set);
    bool changed = set.remove(value);
    index(key, set);
    if (changed) {
      --size;
      if (set.isEmpty())
        data.remove(key);
    }
  }}

  void clear() { synchronized(data) {
    data.clear();
    byCount.clear();
    size = 0;
  }}

  // keys are the values in the byCount multimap
  ItIt<A> keysByPopularityIterator() {
    ret multiSetMapValuesIterator(byCount);
  }
}

Began life as a copy of #1024456