* if there is no such edge.
   */
  @Nullable V value(N node);

  /** Remove {@code node} from the set of predecessors. */
  void removePredecessor(N node);

  /**
   * Remove {@code node} from the set of successors. Returns the value previously associated with
   * the edge connecting the two nodes.
   */
  @CanIgnoreReturnValue

        (N incidentNode) -> EndpointPair.unordered(thisNode, incidentNode));
  }

  @Override
  public @Nullable V value(N node) {
    return adjacentNodeValues.get(node);
  }

  @Override
  public void removePredecessor(N node) {
    @SuppressWarnings("unused")
    V unused = removeSuccessor(node);
  }

  @Override
  public @Nullable V removeSuccessor(N node) {
    return adjacentNodeValues.remove(node);
  }

    }
    return (V) value;
  }

  @Override
  public void removePredecessor(N node) {
    checkNotNull(node);

    Object previousValue = adjacentNodeValues.get(node);
    boolean removedPredecessor;

    if (previousValue == PRED) {
      adjacentNodeValues.remove(node);
      removedPredecessor = true;
    } else if (previousValue instanceof PredAndSucc) {