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) {
    return adjacentNodeValues.put(node, value);
  }

   * becomes a predecessor. Associates {@code value} with the edge connecting the two nodes. Returns
   * the value previously associated with the edge connecting the two nodes.
   */
  @CanIgnoreReturnValue
  @Nullable V addSuccessor(N node, V value);