addEdge(N1, N2, E12);
    assertThat(network.adjacentNodes(N1)).containsExactlyElementsIn(adjacentNodes);
  }

  @Override
  public void adjacentEdges_checkReturnedSetMutability() {
    addEdge(N1, N2, E12);
    Set<String> adjacentEdges = network.adjacentEdges(E12);
    try {
      adjacentEdges.add(E23);
      fail(ERROR_MODIFIABLE_COLLECTION);