if (!allowsSelfLoops && nodeU == nodeV) { // just an optimization, only check reference equality
      return ImmutableSet.of();
    }
    checkArgument(containsNode(nodeV), NODE_NOT_IN_GRAPH, nodeV);
    return nodePairInvalidatableSet(connectionsU.edgesConnecting(nodeV), nodeU, nodeV);
  }

  @Override
  public Set<E> inEdges(N node) {

    if (connections == null) {
      checkNotNull(node);
      throw new IllegalArgumentException("Node " + node + " is not an element of this graph.");
    }
    return connections;
  }

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

  private final boolean hasEdgeConnectingInternal(N nodeU, N nodeV) {
    GraphConnections<N, V> connectionsU = nodeConnections.get(nodeU);