}
    return node;
  }

  @Override
  public void addInEdge(E edge, N node, boolean isSelfLoop) {
    if (!isSelfLoop) {
      addOutEdge(edge, node);
    }
  }

  @Override
  public void addOutEdge(E edge, N node) {
    super.addOutEdge(edge, node);
    Multiset<N> adjacentNodes = getReference(adjacentNodesReference);
    if (adjacentNodes != null) {
      checkState(adjacentNodes.add(node));

    return requireNonNull(previousNode);
  }

  @Override
  public void addInEdge(E edge, N node, boolean isSelfLoop) {
    if (!isSelfLoop) {
      addOutEdge(edge, node);
    }
  }

  @Override
  public void addOutEdge(E edge, N node) {
    N previousNode = incidentEdgeMap.put(edge, node);
    checkState(previousNode == null);
  }

    return requireNonNull(previousNode);
  }

  @Override
  public void addInEdge(E edge, N node, boolean isSelfLoop) {
    if (!isSelfLoop) {
      addOutEdge(edge, node);
    }
  }

  @Override
  public void addOutEdge(E edge, N node) {
    N previousNode = incidentEdgeMap.put(edge, node);
    checkState(previousNode == null);
  }

    Multiset<N> predecessors = getReference(predecessorsReference);
    if (predecessors != null) {
      checkState(predecessors.add(node));
    }
  }

  @Override
  public void addOutEdge(E edge, N node) {
    super.addOutEdge(edge, node);
    Multiset<N> successors = getReference(successorsReference);
    if (successors != null) {
      checkState(successors.add(node));
    }
  }

    Multiset<N> predecessors = getReference(predecessorsReference);
    if (predecessors != null) {
      checkState(predecessors.add(node));
    }
  }

  @Override
  public void addOutEdge(E edge, N node) {
    super.addOutEdge(edge, node);
    Multiset<N> successors = getReference(successorsReference);
    if (successors != null) {
      checkState(successors.add(node));
    }
  }

    if (isSelfLoop) {
      checkPositive(++selfLoopCount);
    }
    N previousNode = inEdgeMap.put(edge, node);
    checkState(previousNode == null);
  }

  @Override
  public void addOutEdge(E edge, N node) {
    checkNotNull(edge);
    checkNotNull(node);

    N previousNode = outEdgeMap.put(edge, node);
    checkState(previousNode == null);
  }

   */
  void addInEdge(E edge, N node, boolean isSelfLoop);

  /** Add {@code edge} to the set of outgoing edges. Implicitly adds {@code node} as a successor. */
  void addOutEdge(E edge, N node);