/**
   * Add {@code node} as a predecessor to the origin node. In the case of an undirected graph, it
   * also becomes a successor. Associates {@code value} with the edge connecting the two nodes.
   */
  void addPredecessor(N node, V value);

  /**
   * Add {@code node} as a successor to the origin node. In the case of an undirected graph, it also

    V unused = removeSuccessor(node);
  }

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

  @Override
  public void addPredecessor(N node, V value) {
    @SuppressWarnings("unused")
    V unused = addSuccessor(node, value);
  }

  @Override
  public @Nullable V addSuccessor(N node, V value) {