public boolean putEdge(N nodeU, N nodeV) {
    return backingValueGraph.putEdgeValue(nodeU, nodeV, Presence.EDGE_EXISTS) == null;
  }

  @Override
  public boolean putEdge(EndpointPair<N> endpoints) {
    validateEndpoints(endpoints);
    return putEdge(endpoints.nodeU(), endpoints.nodeV());
  }

  @Override
  public boolean removeNode(N node) {
    return backingValueGraph.removeNode(node);
  }

  @Override

      checkPositive(++edgeCount);
    }
    return previousValue;
  }

  @Override
  @CanIgnoreReturnValue
  public @Nullable V putEdgeValue(EndpointPair<N> endpoints, V value) {
    validateEndpoints(endpoints);
    return putEdgeValue(endpoints.nodeU(), endpoints.nodeV(), value);
  }

  @Override
  @CanIgnoreReturnValue
  public boolean removeNode(N node) {
    checkNotNull(node, "node");

    return edgeValueOrDefaultInternal(checkNotNull(nodeU), checkNotNull(nodeV), defaultValue);
  }

  @Override
  public @Nullable V edgeValueOrDefault(EndpointPair<N> endpoints, @Nullable V defaultValue) {
    validateEndpoints(endpoints);
    return edgeValueOrDefaultInternal(endpoints.nodeU(), endpoints.nodeV(), defaultValue);
  }

  @Override
  protected long edgeCount() {
    return edgeCount;
  }

    edgeToReferenceNode.put(edge, nodeU);
    return true;
  }

  @Override
  @CanIgnoreReturnValue
  public boolean addEdge(EndpointPair<N> endpoints, E edge) {
    validateEndpoints(endpoints);
    return addEdge(endpoints.nodeU(), endpoints.nodeV(), edge);
  }

  @Override
  @CanIgnoreReturnValue
  public boolean removeNode(N node) {
    checkNotNull(node, "node");