docs: public static <E extends Comparable> void retainTopKeyComparable(Counter<E> c, int num) {
      int numToPurge = c.size() - num;
      if (numToPurge <= 0) {
        return;
      }
  
      List<E> l = Counters.toSortedListKeyComparable(c);
      Collections.reverse(l);
      for (int i = 0; i < numToPurge; i++) {
        c.remove(l.get(i));
      }
    }