docs: public List<T> descendingKeys() {
          List<T> list = new ArrayList<T>(map.keySet());
  
          sort(list, new Comparator<T>() {
              @Override
              public int compare(T o1, T o2) {
                  MutableLong l1 = map.get(o1);
                  MutableLong l2 = map.get(o2);
                  return compare(l2.value, l1.value);
              }
  
              private int compare(long x, long y) {
                  return (x < y) ? -1 : ((x == y) ? 0 : 1);
              }
          });
  
          return list;
      }