docs: public static <E> Set<E> retainBelow(Counter<E> counter, double countMaxThreshold) {
      Set<E> removed = new HashSet<E>();
      for (E key : counter.keySet()) {
        if (counter.getCount(key) > countMaxThreshold) {
          removed.add(key);
        }
      }
      for (E key : removed) {
        counter.remove(key);
      }
      return removed;
    }