}

  @Override
  @CanIgnoreReturnValue
  public boolean addEdge(N nodeU, N nodeV, E edge) {
    checkNotNull(nodeU, "nodeU");
    checkNotNull(nodeV, "nodeV");
    checkNotNull(edge, "edge");

    if (containsEdge(edge)) {
      EndpointPair<N> existingIncidentNodes = incidentNodes(edge);
      EndpointPair<N> newIncidentNodes = EndpointPair.of(this, nodeU, nodeV);
      checkArgument(

    }
    return referenceNode;
  }

  final boolean containsNode(N node) {
    return nodeConnections.containsKey(node);
  }

  final boolean containsEdge(E edge) {
    return edgeToReferenceNode.containsKey(edge);
  }

    }
    return referenceNode;
  }

  final boolean containsNode(N node) {
    return nodeConnections.containsKey(node);
  }

  final boolean containsEdge(E edge) {
    return edgeToReferenceNode.containsKey(edge);
  }