return adjacentNodeValues.remove(node);
  }

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

  @Override
  @CheckForNull
  public V addSuccessor(N node, V value) {
    return adjacentNodeValues.put(node, value);
  }

    return adjacentNodeValues.remove(node);
  }

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

  @Override
  @CheckForNull
  public V addSuccessor(N node, V value) {
    return adjacentNodeValues.put(node, value);
  }

   * the value previously associated with the edge connecting the two nodes.
   */
  @CanIgnoreReturnValue
  @CheckForNull
  V addSuccessor(N node, V value);