implements Serializable {
    final Map<E, Integer> backingMap = new HashMap<>();

    @Override
    public int size() {
      return Multisets.linearTimeSizeImpl(this);
    }

    @Override
    public void clear() {
      throw new UnsupportedOperationException();
    }

    @Override
    public int count(@Nullable Object element) {

   * you override {@link #entrySet}, you may wish to override {@link #size} to forward to this
   * implementation.
   *
   * @since 7.0
   */
  protected int standardSize() {
    return Multisets.linearTimeSizeImpl(this);
  }

  /**
   * A sensible, albeit inefficient, definition of {@link #equals} in terms of {@code
   * entrySet().size()} and {@link #count}. If you override either of these methods, you may wish to