* Returns the entry of the last element in this multiset, or {@code null} if this multiset is
   * empty.
   */
  @CheckForNull
  Entry<E> lastEntry();

  /**
   * Returns and removes the entry associated with the lowest element in this multiset, or returns
   * {@code null} if this multiset is empty.
   */
  @CheckForNull
  Entry<E> pollFirstEntry();

  /**