*
     * @param element the element whose occurrences should all be removed
     * @return the number of occurrences successfully removed, possibly zero
     */
    private int removeAllOccurrences(@Nullable Object element) {
      try {
        return unbox(countMap.remove(element));
      } catch (NullPointerException | ClassCastException e) {
        return 0;
      }
    }

    /**