count = vertices.size() + vertices.size() / 2;
        }

        checkEdges(count);
    }

    private void checkEdges(int nEdges) {
        if (incidentEdges == null) {
            incidentEdges = new HashMap<>(nEdges);
        }
        if (excidentEdges == null) {
            excidentEdges = new HashMap<>(nEdges);
        }
    }

    addNode(N1);
    Set<EndpointPair<Integer>> incidentEdges = graph.incidentEdges(N1);
    assertThrows(
        UnsupportedOperationException.class,
        () -> incidentEdges.add(EndpointPair.unordered(N1, N2)));
    putEdge(N1, N2);
    assertThat(incidentEdges).containsExactlyElementsIn(graph.incidentEdges(N1));
  }

  @Test
  public void predecessors_oneEdge() {
    putEdge(N1, N2);

    addNode(N1);
    Set<EndpointPair<Integer>> incidentEdges = graph.incidentEdges(N1);
    assertThrows(
        UnsupportedOperationException.class,
        () -> incidentEdges.add(EndpointPair.unordered(N1, N2)));
    putEdge(N1, N2);
    assertThat(incidentEdges).containsExactlyElementsIn(graph.incidentEdges(N1));
  }

  @Test
  public void predecessors_oneEdge() {
    putEdge(N1, N2);

  @Test
  public void incidentEdges_checkReturnedSetMutability() {
    addNode(N1);
    Set<String> incidentEdges = network.incidentEdges(N1);
    assertThrows(UnsupportedOperationException.class, () -> incidentEdges.add(E12));
    addEdge(N1, N2, E12);
    assertThat(network.incidentEdges(N1)).containsExactlyElementsIn(incidentEdges);
  }

  @Override
  @Test
  public void adjacentNodes_checkReturnedSetMutability() {

    assume().that(graphIsMutable()).isTrue();

    addNode(N1);
    Set<String> incidentEdges = network.incidentEdges(N1);
    assertThrows(UnsupportedOperationException.class, () -> incidentEdges.add(E12));
    addEdge(N1, N2, E12);
    assertThat(network.incidentEdges(N1)).containsExactlyElementsIn(incidentEdges);
  }

  @Override
  @Test
  public void adjacentNodes_checkReturnedSetMutability() {

    assume().that(graphIsMutable()).isTrue();

    addNode(N1);
    Set<String> incidentEdges = network.incidentEdges(N1);
    assertThrows(UnsupportedOperationException.class, () -> incidentEdges.add(E12));
    addEdge(N1, N2, E12);
    assertThat(network.incidentEdges(N1)).containsExactlyElementsIn(incidentEdges);
  }

  @Override
  @Test
  public void adjacentNodes_checkReturnedSetMutability() {

  @Test
  public void incidentEdges_checkReturnedSetMutability() {
    addNode(N1);
    Set<String> incidentEdges = network.incidentEdges(N1);
    assertThrows(UnsupportedOperationException.class, () -> incidentEdges.add(E12));
    addEdge(N1, N2, E12);
    assertThat(network.incidentEdges(N1)).containsExactlyElementsIn(incidentEdges);
  }

  @Override
  @Test
  public void adjacentNodes_checkReturnedSetMutability() {

  /**
   * Returns the count of {@code node}'s {@link #incidentEdges(Object) incident edges}, counting
   * self-loops twice (equivalently, the number of times an edge touches {@code node}).
   *
   * <p>For directed networks, this is equal to {@code inDegree(node) + outDegree(node)}.
   *
   * <p>For undirected networks, this is equal to {@code incidentEdges(node).size()} + (number of
   * self-loops incident to {@code node}).

   *         <li>{@code incidentEdges(node)}: Edge insertion order
   *       </ul>
   *   <li>For {@link Network}:
   *       <ul>
   *         <li>{@code adjacentNodes(node)}: Stable order
   *         <li>{@code predecessors(node)}: Connecting edge insertion order
   *         <li>{@code successors(node)}: Connecting edge insertion order
   *         <li>{@code incidentEdges(node)}: Stable order

   *         <li>{@code incidentEdges(node)}: Edge insertion order
   *       </ul>
   *   <li>For {@link Network}:
   *       <ul>
   *         <li>{@code adjacentNodes(node)}: Stable order
   *         <li>{@code predecessors(node)}: Connecting edge insertion order
   *         <li>{@code successors(node)}: Connecting edge insertion order
   *         <li>{@code incidentEdges(node)}: Stable order