addEdge(N2, N3, E23);
    addEdge(N3, N1, E31);
    addEdge(N3, N4, E34);
    assertThat(network.adjacentEdges(E12)).containsExactly(E31, E23);
  }

  @Test
  public void adjacentEdges_noAdjacentEdges() {
    addEdge(N1, N2, E12);
    addEdge(N3, N4, E34);
    assertThat(network.adjacentEdges(E12)).isEmpty();
  }

  @Test
  public void adjacentEdges_edgeNotInGraph() {