// persistable and synchronized
sclass MRUAndAllTimeTop_naive<A> {
  Map<A, Bool> mru = syncMRUCache(10); // most recently used
  new MultiSet<A> allTimeTop;
  int size;
  
  // increase a's score in all time top and move to front int MRU
  void add(A a) {
    mru.remove(a); // force re-insertion
    mru.put(a, true);
    allTimeTop.add(a);
  }
  
  void addAll(Iterable<A> l) { fOr (A a : l) add(a); }
  
  // may count elements twice
  int size() {
    ret l(mru) + allTimeTop.uniqueSize();
  }
  
  // interleave latest and most often accessed elements
  ItIt<A> mixedIterator() {
    ret uniqueIterator(roundRobinCombinedIterator(
      reversedIterator(keysList(mru)),
      iterator(allTimeTop.highestFirst())));
  }
}

Began life as a copy of #1024856