return true;
    }

    // value changed
    return false;
  }

  /**
   * Atomically remove {@code key} from the map iff its associated value is 0.
   *
   * @since 20.0
   */
  @CanIgnoreReturnValue
  public boolean removeIfZero(K key) {
    return remove(key, 0);
  }

  /**

     * the elem field without a null check by calling getElement().
     */
    @CheckForNull private final E elem;

    // elemCount is 0 iff this node has been deleted.
    private int elemCount;

    private int distinctElements;
    private long totalCount;
    private int height;
    @CheckForNull private AvlNode<E> left;

   * @since 27.1
   */
  @CheckForNull
  V edgeValueOrDefault(EndpointPair<N> endpoints, @CheckForNull V defaultValue);

  //
  // ValueGraph identity
  //

  /**
   * Returns {@code true} iff {@code object} is a {@link ValueGraph} that has the same elements and
   * the same structural relationships as those in this graph.
   *
   * <p>Thus, two value graphs A and B are equal if <b>all</b> of the following are true:

  protected final void validateEndpoints(EndpointPair<?> endpoints) {
    checkNotNull(endpoints);
    checkArgument(isOrderingCompatible(endpoints), ENDPOINTS_MISMATCH);
  }

  /**
   * Returns {@code true} iff {@code endpoints}' ordering is compatible with the directionality of
   * this graph.
   */
  protected final boolean isOrderingCompatible(EndpointPair<?> endpoints) {
    return endpoints.isOrdered() == this.isDirected();

  /*
   * TODO: consider adding MultisetFeatures.SUPPORTS_SET_COUNT. Currently we
   * assume that using setCount() to increase the count is permitted iff add()
   * is permitted and similarly for decrease/remove(). We assume that a
   * setCount() no-op is permitted if either add() or remove() is permitted,
   * though we also allow it to "succeed" if neither is permitted.
   */

  /*
   * TODO: consider adding MultisetFeatures.SUPPORTS_SET_COUNT. Currently we
   * assume that using setCount() to increase the count is permitted iff add()
   * is permitted and similarly for decrease/remove(). We assume that a
   * setCount() no-op is permitted if either add() or remove() is permitted,
   * though we also allow it to "succeed" if neither is permitted.
   */

   *
   * @since 27.1
   */
  @Override
  boolean hasEdgeConnecting(EndpointPair<N> endpoints);

  //
  // Graph identity
  //

  /**
   * Returns {@code true} iff {@code object} is a {@link Graph} that has the same elements and the
   * same structural relationships as those in this graph.
   *
   * <p>Thus, two graphs A and B are equal if <b>all</b> of the following are true:
   *

   *
   * @since 27.1
   */
  boolean hasEdgeConnecting(EndpointPair<N> endpoints);

  //
  // Network identity
  //

  /**
   * Returns {@code true} iff {@code object} is a {@link Network} that has the same elements and the
   * same structural relationships as those in this network.
   *
   * <p>Thus, two networks A and B are equal if <b>all</b> of the following are true:
   *