// TODO: make persistable
sclass MRUAndAllTimeTop<A> {
  new MRUCache<A, Bool> mru; // most recently used
  new MultiSet<A> allTimeTop;
  
  // increase a's score in all time top and move to front int MRU
  void add(A a) {
    mru.put(a, true);
    allTimeTop.add(a);
  }
  
  // interleave latest and most often accessed elements
  ItIt<A> mixedIterator() {
    ret roundRobinCombinedIterator(
      reversedIterator(keysList(mru)),
      iterator(allTimeTop.highestFirst()));
  }
}