//
  // Element-level accessors
  //

  /**
   * Returns a live view of the nodes which have an incident edge in common with {@code node} in
   * this graph.
   *
   * <p>This is equal to the union of {@link #predecessors(Object)} and {@link #successors(Object)}.
   *
   * <p>If {@code node} is removed from the graph after this method is called, the {@code Set}

        checkArgument(edges().contains(edge));
        N nodeU = edge.nodeU();
        N nodeV = edge.nodeV();
        Set<EndpointPair<N>> endpointPairIncidentEdges =
            Sets.union(incidentEdges(nodeU), incidentEdges(nodeV));
        return nodePairInvalidatableSet(
            Sets.difference(endpointPairIncidentEdges, ImmutableSet.of(edge)), nodeU, nodeV);
      }

      @Override

      assertThat(removalNotifications.get("b" + i)).isEqualTo("b" + i);
    }

    // Each of the values added to the map should either still be there, or have seen a removal
    // notification.
    assertThat(Sets.union(cache.asMap().keySet(), removalNotifications.keySet()))
        .isEqualTo(expectedKeys);
    assertThat(cache.asMap().keySet()).containsNoneIn(removalNotifications.keySet());
    threadPool.shutdown();

  // Element-level accessors
  //

  /**
   * Returns a live view of the edges whose {@link #incidentNodes(Object) incident nodes} in this
   * network include {@code node}.
   *
   * <p>This is equal to the union of {@link #inEdges(Object)} and {@link #outEdges(Object)}.
   *
   * <p>If {@code node} is removed from the network after this method is called, the {@code Set}

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