addEdge(N1, N2, E12);
    Set<String> adjacentEdges = network.adjacentEdges(E12);
    try {
      adjacentEdges.add(E23);
      fail(ERROR_MODIFIABLE_COLLECTION);
    } catch (UnsupportedOperationException e) {
      addEdge(N2, N3, E23);
      assertThat(network.adjacentEdges(E12)).containsExactlyElementsIn(adjacentEdges);
    }
  }

  @Override
  @Test

    addEdge(N1, N2, E12);
    Set<String> adjacentEdges = network.adjacentEdges(E12);
    try {
      adjacentEdges.add(E23);
      fail(ERROR_MODIFIABLE_COLLECTION);
    } catch (UnsupportedOperationException e) {
      addEdge(N2, N3, E23);
      assertThat(network.adjacentEdges(E12)).containsExactlyElementsIn(adjacentEdges);
    }
  }

  @Override
  @Test

    addEdge(N1, N2, E12);
    Set<String> adjacentEdges = network.adjacentEdges(E12);
    try {
      adjacentEdges.add(E23);
      fail(ERROR_MODIFIABLE_COLLECTION);
    } catch (UnsupportedOperationException e) {
      addEdge(N2, N3, E23);
      assertThat(network.adjacentEdges(E12)).containsExactlyElementsIn(adjacentEdges);
    }
  }

  @Override
  @Test

              }
            };
        return nodeInvalidatableSet(incident, node);
      }

      @Override
      public Set<EndpointPair<N>> adjacentEdges(EndpointPair<N> edge) {
        checkArgument(edges().contains(edge));
        N nodeU = edge.nodeU();
        N nodeV = edge.nodeV();
        Set<EndpointPair<N>> endpointPairIncidentEdges =

  }

  @Override
  public int outDegree(N node) {
    return isDirected() ? outEdges(node).size() : degree(node);
  }

  @Override
  public Set<E> adjacentEdges(E edge) {
    EndpointPair<N> endpointPair = incidentNodes(edge); // Verifies that edge is in this network.
    Set<E> endpointPairIncidentEdges =

   *       behavior is undefined
   * </ul>
   *
   * @throws IllegalArgumentException if {@code edge} is not an element of this network
   */
  Set<E> adjacentEdges(E edge);

  /**
   * Returns a live view of the set of edges that each directly connect {@code nodeU} to {@code
   * nodeV}.
   *