addEdge(N1, N2, E12_A);
    addEdge(N1, N2, E12_B);
    addEdge(N3, N4, E34);

    assertThat(network.adjacentEdges(E12)).containsExactly(E12_A, E12_B);
  }

  @Test
  public void edgesConnecting_disconnectedNodes() {
    addNode(N1);
    addNode(N2);
    assertThat(network.edgesConnecting(N1, N2)).isEmpty();
  }

  @Test
  public void edgesConnecting_nodesNotInGraph() {
    addNode(N1);