/**
  * This test checks an implementation dependent feature. It tests that the method {@code addEdge}
  * will silently add the missing nodes to the builder, then add the edge connecting them. We are
  * not using the proxy methods here as we want to test {@code addEdge} when the end-points are not
  * elements of the graph.
  */
 @Test
 public void addEdge_builder_nodesNotInGraph() {
   addNode(N1);
   assertTrue(addEdge(E15, N1, N5));
   assertTrue(addEdge(E41, N4, N1));
   assertTrue(addEdge(E23, N2, N3));
   assertThat(immutableGraph.nodes()).containsExactly(N1, N5, N4, N2, N3).inOrder();
   assertThat(immutableGraph.edges()).containsExactly(E15, E41, E23).inOrder();
   assertThat(immutableGraph.edgesConnecting(N1, N5)).containsExactly(E15);
   assertThat(immutableGraph.edgesConnecting(N4, N1)).containsExactly(E41);
   assertThat(immutableGraph.edgesConnecting(N2, N3)).containsExactly(E23);
   assertThat(immutableGraph.edgesConnecting(N3, N2)).containsExactly(E23);
 }
 @Test
 public void addEdge_builder_existingNodes() {
   // Adding nodes initially for safety (insulating from possible future
   // modifications to proxy methods)
   addNode(N1);
   addNode(N2);
   assertTrue(addEdge(E12, N1, N2));
   assertThat(immutableGraph.edges()).contains(E12);
   assertThat(immutableGraph.edgesConnecting(N1, N2)).containsExactly(E12);
   assertThat(immutableGraph.edgesConnecting(N2, N1)).containsExactly(E12);
 }
 @Test
 public void edgesConnecting_oneEdge() {
   addEdge(E12, N1, N2);
   assertThat(immutableGraph.edgesConnecting(N1, N2)).containsExactly(E12);
   assertThat(immutableGraph.edgesConnecting(N2, N1)).containsExactly(E12);
 }