return adjacentNodeValues.get(node);
  }

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

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

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