* references to the map or modify it in any way.
   *
   * <p>The returned multiset is serializable if the input map is.
   *
   * @param countMap backing map for storing the elements in the multiset and their counts. It must
   *     be empty.
   * @throws IllegalArgumentException if {@code countMap} is not empty
   * @since 20.0
   */
  public static <E> ConcurrentHashMultiset<E> create(ConcurrentMap<E, AtomicInteger> countMap) {