* undirected edges connect a pair of nodes to each other.
   */
  boolean isDirected();

  /**
   * Returns true if this graph allows self-loops (edges that connect a node to itself). Attempting
   * to add a self-loop to a graph that does not allow them will throw an {@link
   * IllegalArgumentException}.
   */
  boolean allowsSelfLoops();

OkHttp TLS
==========

Approachable APIs for using TLS.

A [`HeldCertificate`][held_certificate] is a certificate and its private key. Use the
[builder][held_certificate_builder] to create a self-signed certificate that a test server can use
for HTTPS:

```java
HeldCertificate localhostCertificate = new HeldCertificate.Builder()
    .addSubjectAlternativeName("localhost")
    .build();
```

        <artifactId>maven-compiler-plugin</artifactId>
        <executions>
          <execution>
            <id>default-compile</id>
            <configuration>
              <compilerArgs combine.children="append" combine.self="append">
                <arg>-XDignore.symbol.file</arg>
              </compilerArgs>
            </configuration>
          </execution>
          <execution>
            <id>compile-java9</id>

        <executions>
          <execution>
            <id>default-compile</id>
            <configuration>
              <compilerArgs combine.children="append" combine.self="append">
                <arg>-XDignore.symbol.file</arg>
              </compilerArgs>
            </configuration>
          </execution>
          <execution>
            <id>compile-java9</id>

        <executions>
          <execution>
            <id>default-compile</id>
            <configuration>
              <compilerArgs combine.children="append" combine.self="append">
                <arg>-XDignore.symbol.file</arg>
              </compilerArgs>
            </configuration>
          </execution>
          <execution>
            <id>compile-java9</id>

      // recordSslDebug = true
    }

  // https://tools.ietf.org/html/rfc6066#page-6 specifies a FQDN is required.
  val hostname = "local.host"
  private val handshakeCertificates =
    run {
      // Generate a self-signed cert for the server to serve and the client to trust.
      val heldCertificate =
        HeldCertificate
          .Builder()
          .commonName(hostname)
          .addSubjectAlternativeName(hostname)

   * 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}).
   *

  // here too.
  static final String ERROR_MODIFIABLE_SET = "Set returned is unexpectedly modifiable";
  static final String ERROR_SELF_LOOP = "self-loops are not allowed";
  static final String ERROR_ADDED_SELF_LOOP = "Should not be allowed to add a self-loop edge.";

  /** Creates and returns an instance of the graph to be tested. */
  abstract Graph<Integer> createGraph();

  /**

  // here too.
  static final String ERROR_MODIFIABLE_SET = "Set returned is unexpectedly modifiable";
  static final String ERROR_SELF_LOOP = "self-loops are not allowed";
  static final String ERROR_ADDED_SELF_LOOP = "Should not be allowed to add a self-loop edge.";

  /** Creates and returns an instance of the graph to be tested. */
  abstract Graph<Integer> createGraph();

  /**

  private static final String ERROR_PARALLEL_EDGE = "connected by a different edge";
  private static final String ERROR_NEGATIVE_COUNT = "is non-negative";
  static final String ERROR_SELF_LOOP = "self-loops are not allowed";

  @Test
  public void transitiveClosure_directedGraph_addSelfLoopsAlways() {
    MutableGraph<Integer> directedGraph = GraphBuilder.directed().allowsSelfLoops(false).build();