// TODO: make persistable
sclass MRUAndAllTimeTop<A> {
  new MRUCache<A> mru; // most recently used
  new MultiSet<A> allTimeTop;
  
  // interleave latest and most often accessed elements
  ItIt<A> mixedIterator() {
    ret roundRobinCombinedIterator(
      reversedIterator(asArrayList(mru)),
      iterator(allTimeTop.highestFirst()));
  }
}